Joining Method and Friction Stir Welding Method

ABSTRACT

A method of welding that realizes enhancements of airtightness and watertightness at weld area and a method of welding and method of friction stir welding that attain an enhancement of welding operation efficiency/speed. There is provided a method of welding including the first primary welding step of carrying out a friction stir welding on the abutting portion (J 1 ) of metal members ( 1,1 ) from the surface ( 12 ) side of metal member ( 1 ) and the second primary welding step of carrying out a friction stir welding on the abutting portion (J 1 ) from the reverse face ( 13 ) side of the member ( 1 ), wherein in the second primary welding step, the friction stir welding is performed while penetrating a stirring pin (B 2 ) of rotation tool (B) in plasticized region (W 1 ) formed in the first primary welding step.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 12/443,787filed Mar. 31, 2009 which is a national stage of PCT/JP2007/055346 filedMar. 16, 2007, and claims priority of Japanese patent application nos.2006-271243, 2006-271244, 2006-271245, 2006-271246, 2006-271247,2006-271248, and 2006-271249, filed Oct. 2, 2006, respectively.

TECHNICAL FIELD

The present invention relates to a joining method of metal members usingfriction stir welding, and a friction stir welding method.

BACKGROUND ART

As a method for joining metal members to each other, friction stirwelding (FSW) is known (refer to Documents 1 to 9 (Japanese PublishedUnexamined Patent Applications published by the Japan Patent Office)listed below). According to friction stir welding, a rotating tool ismoved along a butting portion between metal members while the rotatingtool is rotated, and by plasticizing and fluidizing the metal of thebutting portion by frictional heat of the rotating tool and the metalmembers, the metal members are joined in solid phase. The rotating toolis generally formed by providing a stirring pin (probe) in a projectingmanner on a lower end surface of a shoulder having a columnar shape.

DOCUMENT

-   1. Japanese Published Unexamined Patent Application No. H11-342481-   2. Japanese Published Unexamined Patent Application No. 2004-358535-   3. Japanese Published Unexamined Patent Application No. 2005-131666-   4. Japanese Published Unexamined Patent Application No. 2003-164980-   5. Japanese Published Unexamined Patent Application No. 2002-248582-   6. Japanese Published Unexamined Patent Application No. 2002-1551-   7. Japanese Published Unexamined Patent Application No. 2005-66669-   8. Japanese Published Unexamined Patent Application No. 2004-154798-   9. Japanese Published Unexamined Patent Application No. 2003-326374

When the thicknesses of the metal members to be joined are larger thanthe length of the stirring pin of the rotating tool, after friction stirwelding is performed from the surface side of the metal members,friction stir welding may be performed from the back surface side (forexample, refer to Documents 1 to 3). By increasing the length of thestirring pin, the metal members can be joined by performing frictionstir welding from only the surface side, however, the load on a drivemeans of the friction stir welding machine increases, so that if theexisting friction stir welding machine cannot cope with this, thefriction stir welding machine must be altered or a large-sized frictionstir welding machine must be introduced.

When a butting portion between the metal members is friction stirwelded, as described in Document 2, a so-called tunnel-like defect mayoccur along the traveling direction of the rotating tool. There are somecases where the tunnel-like defect does not affect the quality of thejoined portion, however, when high airtightness and water-tightness arerequired, the tunnel-like defect is divided to be discontinuous.Document 2 discloses a technique for joining metal members by forming adepression portion on a butting portion between the metal members bybutting the metal members having step portions on their end surfaces andperforming friction stir welding abutting portion between a connectingplate fitted in the depression portion and the metal members, and atechnique is also disclosed in which the tunnel-like defect is dividedto be discontinuous by moving the rotating tool across the buttingportion after friction stir welding the butting portion between themetal members.

Documents 4 and 5 disclose a technique for preliminarily joining abutting portion between metal members to be joined before they areprimarily welded by friction stir welding, a technique for disposing tabmembers (splint member) on both sides of the butting portion between themetal members and providing a start position or an end position offriction stir welding for primary welding on the tab members, and atechnique for joining the tab members to the metal members before thebutting portion between the metal members is primarily welded, etc.

Document 6 discloses a technique for preliminarily joining a buttingportion between metal members to be joined by friction stir weldingbefore the butting portion is primarily welded by friction stir welding,and Document 7 discloses a technique for providing a start position oran end position of friction stir welding for primary welding on tabmembers disposed on both sides of the butting portion between the metalmembers, and a technique for joining the tab members to the metalmembers by friction stir welding before the butting portion between themetal members is primarily welded, etc.

Document 7 and Document 8, etc., disclose a technique for providing astart position or an end position of friction stir welding on a tabmember disposed lateral to the butting portion between metal members tobe joined.

Document 9 discloses a friction stir welding method in which a pilotbore is made at a start position of friction stir welding and frictionstir welding is started from this pilot bore.

Friction stir welding conditions such as the rotating speed and feedrate (movement speed) of the rotating tool are appropriately setaccording to the materials and thicknesses of the metal members to befriction stir welded, and various friction stir welding conditions aredisclosed in Documents 2, 3, 5, 6, and 9, and in all of these documents,the rotating speed of the rotating tool is maintained constant and therotating tool is moved at a constant feed rate from the time when therotating tool is inserted (pressed) into the start position to the timewhen the rotating tool is withdrawn from the end position.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In the joining methods of Documents 1 to 3, fine joining defects mayremain at the boundary between a region plasticized by friction stirwelding from the surface side (hereinafter, may be referred to as“surface plasticized region”) and a region plasticized by friction stirwelding from the back surface side (hereinafter, may be referred to as“back plasticized region”). If such joining defects are continued, thismay deteriorate the airtightness and water-tightness of the joinedportion and this is undesirable. In Documents 1 to 3, the surfaceplasticized region and the back plasticized region are made to contacteach other or slightly overlap at the central portion in the thicknessdirection of the metal members to join the metal members entirely in thethickness direction, so that the joining strength does not becomeinsufficient.

From this point of view, an object of the present invention is toprovide a joining method capable of improving the airtightness andwater-tightness of a joined portion, and further provide a joiningmethod and a friction stir welding method which can improve joiningefficiency and speed.

Means for Solving the Problems

A joining method according to claim 1 for solving the above-describedproblem includes: a first primary welding process in which a buttingportion between metal members is friction stir welded from the surfaceside of the metal members; and a second primary welding process in whichthe butting portion is friction stir welded from the back surface sideof the metal members, wherein a sum of a length of a stirring pin of arotating tool to be used in the first primary welding process and alength of a stirring pin of the rotating tool to be used in the secondprimary welding process is set to not less than the thickness of themetal members at the butting portion.

In other words, according to the joining method of the presentinvention, in the second primary welding process, friction stir weldingis performed while the stirring pin of the rotating tool is insertedinto a plasticized region formed through the first primary weldingprocess.

Accordingly, a deep portion of the plasticized region (surfaceplasticized region) formed through the first primary welding process isfriction stir welded again by the stirring pin of the rotating tool inthe second primary welding process, so that even when joining defects,etc., are formed at the deep portion of the surface plasticized region,the joining defects, etc., can be corrected, and eventually, theairtightness and water-tightness of the joined portion can be improved.

The plasticization reaches a position deeper than the tip end of thestirring pin, so that even when the stirring pin of the rotating tool tobe used for the friction stir welding from the back surface side isshort and does not reach the surface plasticized region as in theconventional technique described above, the surface plasticized regionand the back plasticized region can be made to contact each other orslightly overlap at the central portion in the thickness direction ofthe metal members, however, joining defects, etc., formed at the deepportion of the surface plasticized region cannot be reliably corrected.On the other hand, in the present invention, the stirring pin isinserted into the surface plasticized region and the region is directlyfriction stir welded in the second primary welding process, so that thejoining defects formed in the surface plasticized region can besufficiently reliably corrected.

In the present invention, the length of a stirring pin of a rotatingtool to be used in the first primary welding process is set to not lessthan ½ of the thickness of the metal members at the butting portion, andthe length of a stirring pin of a rotating tool to be used in the secondprimary welding process is set to not less than ½ of the thickness ofthe metal members at the butting portion. By thus setting, the surfaceplasticized region and the back plasticized region overlap at thecentral portion in the thickness direction of the metal members, and thedifference in sectional area between the surface plasticized region andthe back plasticized region is reduced, so that the joined portionbecomes uniform in quality. It is a matter of course that the lengths ofthe stirring pins of the rotating tools are less than the thickness.

By setting the length of the stirring pin of the rotating tool to beused in the second primary welding process to be equal to the length ofthe stirring pin of the rotating tool to be used in the first primarywelding process, the surface plasticized region and the back plasticizedregion become equal to each other, so that the joined portion becomesmore uniform in quality, and in addition, the same rotating tool can beused in the first primary welding process and the second primary weldingprocess, so that the working efficiency can be improved.

The length L₁ of the stirring pin of the rotating tool to be used in thefirst primary welding process and the length L₂ of the stirring pin ofthe rotating tool to be used in the second primary welding process areset so as to satisfy the relation of 1.01≦(L₁+L₂)/t≦1.10 where t is thethickness of the metal members at the butting portion. When (L₁+L₂)/t isset to not less than 1.01, even if the metal members have a dimensionaltolerance, etc., the stirring pin can be reliably inserted into thesurface plasticized region in the second primary welding process. If(L₁+L₂)/t is more than 1.10, the rotating tools become excessively largeand the load on the friction stir welding machine increases, so thatpreferably, (L₁+L₂)/t is set to not more than 1.10.

In the present invention, the length of the stirring pin of the rotatingtool to be used in the first primary welding process is set to not morethan ¾ of the thickness of the metal members at the butting portion, andthe length of the stirring pin of the rotating tool to be used in thesecond primary welding process is set to not more than ¾ of thethickness of the metal members at the butting portion. Accordingly, abacking material becomes unnecessary when friction stir welding, so thatthe working efficiency can be improved.

When the stirring pin is withdrawn upward at the end position offriction stir welding in the first primary welding process, a pull-outhole in substantially the same shape as the stirring pin is inevitablyformed, and if the movement route of the stirring pin of the rotatingtool used in the second primary welding process and the pull-out holeoverlap, plasticized and fluidized metal may flow into the pull-out holeand cause joining defects. Therefore, when the pull-out hole formedthrough the first primary welding process remains as it is, the frictionstir welding route in the second primary welding process is preferablyset so as to avoid the pull-out hole, and along this route, the stirringpin of the rotating tool to be used in the second primary weldingprocess is moved.

Even when the stirring pin of the rotating tool to be used in the secondprimary welding process does not pass through the pull-out hole of thefirst primary welding process, if the spacing distance between these issmall, plasticized and fluidized metal may be extruded into the pull-outhole and cause joining defects, so that, more preferably, the shortestdistance between the end position of friction stir welding of the firstprimary welding process and a movement locus of the stirring pin of therotating tool used in the second primary welding process is set to notless than the outer diameter of the shoulder of a rotating tool to beused in the second primary welding process.

Before executing the first primary welding process, a preliminaryjoining process may be executed to preliminarily join the buttingportion between the metal members. The preliminary joining means is notespecially limited, and it can be performed by welding or friction stirwelding, however, when the preliminary joining process is performed byfriction stir welding, preferably, the butting portion is friction stirwelded from the surface side of the metal members by using a preliminaryjoining rotating tool smaller than the rotating tool to be used in thefirst primary welding process. Accordingly, when the surface side of themetal members is friction stir welded in the first primary weldingprocess, separation hardly occurs in the butting portion, so thatjoining with high accuracy is possible. When the butting portion iscooled with water when executing the first primary welding process, thecooling water enters between the metal members and may produce an oxidefilm on the joining surfaces, however, by closing the gap on the surfaceside of the metal members by executing the preliminary joining process,the cooling water hardly enters between the metal members, so that thequality of the joined portion can be improved.

Before executing the second primary welding process, the preliminaryjoining process may be executed. In this case, preferably, the buttingportion is friction stir welded from the back surface side of the metalmembers by using a preliminary joining rotating tool smaller than therotating tool to be used in the second primary welding process.

The preliminary joining and joining of tab members to the metal membersto be performed before the primary welding are only preparatory joiningfor the primary welding, so that it is demanded to improve theefficiency and speed of these, however, the techniques of Documents 4 to7 described above cannot improve the efficiency and speed of thesepreparatory joining operations.

For example, as in the case of Documents 4 and 5, when the preliminaryjoining of the metal members and joining of tab members to the metalmembers are performed by welding, manual operations increase, so thatthis deteriorates the working efficiency and is not suitable in the casewhere many portions are to be welded or in the case of mass production.

As in Documents 6 and 7, when the preparatory joining such as thepreliminary joining and joining of tab members is performed by frictionstir welding, a friction stir welding machine is used, and this makesthe efficiency and speed higher than in the case of manual welding,however, it takes time to insert and extract a rotating tool, and if thenumber of insertions and withdrawals of the rotating tool increases,this hinders improvement in efficiency and speed of the preparatoryjoining operations.

To solve this problem, the joining method of claim 11 is preferablyadopted. The joining method of claim 11 includes: a preparation processin which a first tab member is disposed on one end side of a buttingportion between metal members and a contact surface of the first tabmember is brought into contact with the lateral sides of both metalmembers, and a second tab member is disposed on the other end side ofthe butting portion and a contact surface of the second tab member isbrought into contact with the lateral sides of both metal members; afirst tab member joining process in which a butting portion between thefirst tab member and both metal members is friction stir welded; apreliminary joining process in which the butting portion between themetal members is friction stir welded; a second tab member joiningprocess in which a butting portion between the second tab member andboth metal members is friction stir welded; and a primary weldingprocess in which a butting portion between the metal members which waspreliminarily joined is friction stir welded, wherein the friction stirwelding is not finished at an end point of the first tab member joiningprocess but continued to a start point of the preliminary joiningprocess, and directly shifted to the preliminary joining process.

Accordingly, the withdrawing operation of the rotating tool at theendpoint of the first tab member joining process becomes unnecessary,and further, the inserting operation of the rotating tool at the startpoint of the preliminary joining process becomes unnecessary, so thatthe preparatory joining operations can be improved in efficiency andspeed.

When the friction stir welding is continued to the start point of thepreliminary joining process without withdrawing the rotating tool, it isallowed that the friction stir welding is not finished at the startpoint of the preliminary joining process but continued to an endpoint ofthe preliminary joining process. In this case, all withdrawingoperations of the rotating tool during the preliminary joining processbecome unnecessary, so that the preparatory joining operations can befurther improved in efficiency and speed.

When the friction stir welding is continued to the end point of thepreliminary joining process without withdrawing the rotating tool, it isallowed that the friction stir welding is not finished at the end pointof the preliminary joining process but continued to a start point of thesecond tab member joining process, and directly shifted to the secondtab member joining process. In this case, the withdrawing operation ofthe rotating tool at the end point of the preliminary joining processbecomes unnecessary, and further, the inserting operation of therotating tool at the start point of the second tab member joiningprocess becomes unnecessary, so that the preparatory joining operationscan be further improved in efficiency and speed.

When the friction stir welding is continued to the start point of thesecond tab member joining process without withdrawing the rotating tool,it is allowed that the friction stir welding is not finished at an endpoint of the second tab member joining process but continued from thestart point to the end point of the second tab member joining process.Accordingly, the withdrawing operation of the rotating tool at theendpoint of the second tab member joining process becomes unnecessary,so that the preparatory joining operations can be further improved inefficiency and speed.

Even when the rotating tool is withdrawn in the first tab member joiningprocess, by continuing the friction stir welding to the start point ofthe second tab member joining process without finishing it at the endpoint of the preliminary joining process, and directly shifting it tothe second tab member joining process, the withdrawing operation of therotating tool at the end point of the preliminary joining processbecomes unnecessary, and in addition, the inserting operation of therotating tool at the start point of the second tab member joiningprocess becomes unnecessary, so that the preparatory joining operationscan be further improved in efficiency and speed.

When the rotating tool is rotated clockwise, fine joining defects mayoccur on the left in the traveling direction of the rotating tool.Therefore, when the rotating tool to be used in the first tab memberjoining process is rotated clockwise, the positions of the start pointand the endpoint of the first tab member joining process are set so thatthe metal members are positioned on the right in the traveling directionof the rotating tool. Accordingly, joining defects hardly occur on themetal member side, so that a high-quality joined body can be obtained.

When the rotating tool is rotated counterclockwise, fine joining defectsmay occur on the right in the traveling direction of the rotating tool.Therefore, when the rotating tool to be used in the first tab memberjoining process is rotated counterclockwise, the positions of the startpoint and the end point of the first tab member joining process are setso that the metal members are positioned on the left in the travelingdirection of the rotating tool.

Further, a friction stir welding route from the endpoint of the firsttab member joining process to the start point of the preliminary joiningprocess is set on the first tab member, and a movement locus of therotating tool of the movement from the end point of the first tab memberjoining process to the start point of the preliminary joining process isformed on the first tab member. Accordingly, in the process from the endpoint of the first tab member joining process to the start point of thepreliminary joining process, joining defects hardly occur in the metalmembers, so that a high-quality joined body can be obtained.

The spacing distance between the friction stir welding route in thefirst tab member joining process and the friction stir welding routefrom the end point of the first tab member joining process to the startpoint of the preliminary joining process is preferably secured not lessthan the outer diameter of the shoulder of the rotating tool. In otherwords, preferably, the spacing distance between the movement locusformed when the rotating tool is moved from the start point to the endpoint of the first tab material joining process and the movement locusformed when the rotating tool is moved from the end point of the firsttab member joining process to the start point of the preliminary joiningprocess is secured not less than the outer diameter of the shoulder ofthe rotating tool. Accordingly, even if joining defects occur on themetal member side of the rotating tool in the process from the end pointof the first tab member joining process to the start point of thepreliminary joining process, the joining defects hardly reach the metalmembers, so that a high-quality joined body can be obtained.

When the rotating tool to be used in the second tab member joiningprocess is rotated clockwise, preferably, the metal members arepositioned on the right in the traveling direction of the rotating tool,however, depending on the positional relation between the metal membersand the second tab member, the metal members may be positioned on theleft in the traveling direction. In this case, a turning point isprovided in the friction stir welding route from the start point to theend point of the second tab member joining process, and the positions ofthe start point, turning point, and end point of the second tab memberjoining process are set so that the metal members are positioned on theright in the traveling direction of the rotating tool at least in thefriction stir welding route from the turning point to the end point.Accordingly, even when the metal members are positioned on the left inthe traveling direction of the rotating tool until reaching the turningpoint and joining defects occur on the metal member side, in thesubsequent friction stir welding from the turning point to the endpoint, the metal members are positioned on the right in the travelingdirection of the rotating tool, so that the joining defects arecorrected, and a high-quality joined body can be obtained.

When the rotating tool to be used in the second tab member joiningprocess is rotated counterclockwise, preferably, the metal members arepositioned on the left in the traveling direction of the rotating tool,however, depending on the positional relation between the metal membersand the second tab member, the metal members may be positioned on theright in the traveling direction. In this case, a turning point isprovided in the friction stir welding route from the start point to theendpoint of the second tab member joining process, and the positions ofthe start point, turning point, and endpoint of the second tab memberjoining process are set so that the metal members are positioned on theleft in the traveling direction of the rotating tool at least in thefriction stir welding route from the turning point to the end point.

In addition, a friction stir welding route from the end point of thepreliminary joining process to the start point of the second tab memberjoining process is set on the second tab member, and a movement locus ofthe rotating tool of the movement from the endpoint of the preliminaryjoining process to the start point of the second tab member joiningprocess is formed on the second tab member. Accordingly, in the processfrom the end point of the preliminary joining process to the start pointof the second tab member joining process, joining defects hardly occurin the metal members, so that a high-quality joined body can beobtained.

Preferably, the spacing distance between the friction stir welding routefrom the end point of the preliminary joining process to the start pointof the second tab member joining process and the friction stir weldingroute in the second tab member joining process is secured not less thanthe outer diameter of the shoulder of the rotating tool. In other words,the spacing distance between the movement locus formed when the rotatingtool is moved from the end point of the preliminary joining process tothe start point of the second tab member joining process and themovement locus formed when the rotating tool is moved from the startpoint to the end point of the second tab member joining process issecured not less than the outer diameter of the shoulder of the rotatingtool. Accordingly, in the process from the end point of the preliminaryjoining process to the start point of the second tab member joiningprocess, even if joining defects occur on the metal member side of therotating tool, the joining defects hardly reach the metal members, sothat a high-quality joined body can be obtained.

In the present invention, after a pilot bore is formed at the startposition of the friction stir welding in the primary welding process,the friction stir welding is started from the pilot bore. In otherwords, at a position at which the stirring pin will be inserted, thepilot bore is formed. Accordingly, the load (pressure) of insertion ofthe stirring pin of the primary welding rotating tool to be used in theprimary welding process can be reduced, so that the load on the frictionstir welding machine can be reduced, and further, the insertion speed ofthe primary welding rotating tool can be increased, so that theoperation time required for the primary welding can be shortened.

When the pilot bore is formed, the maximum bore diameter of the pilotbore is set to be smaller than the maximum outer diameter of thestirring pin of the primary welding rotating tool to be used in theprimary welding process, and the stirring pin is press-fitted into thepilot bore while being rotated. Accordingly, the stirring pin comes intocontact with the bore wall of the pilot bore and generates frictionalheat before the shoulder of the primary welding rotating tool comes intocontact with the surfaces of the metal members, etc., so that the timeuntil plasticization and fluidization can be shortened.

When the pilot bore is formed, it is formed in the butting portionbetween the first tab member and the metal members, or the buttingportion between the second tab member and the metal members.Accordingly, a part of the metal plasticized and fluidized when thestirring pin of the primary welding rotating tool is press-fitted intothe pilot bore flows into a fine gap between the tab member and themetal members, and thereafter, the escape of the metal plasticized andfluidized into the gap is reduced, so that joining defects due toinsufficient thickness hardly occur.

When the stirring pin of the primary welding rotating tool ispress-fitted into the pilot bore formed in the butting portion betweenthe tab member and the metal members, a force to separate the tab memberand the metal members acts, and a gap may be created between the tabmember and the metal members, and therefore, when a pilot bore is formedin the butting portion between the first tab member and the metalmembers, before the primary welding rotating tool is inserted into thepilot bore, inner corner portions formed by the metal members and thefirst tab member are preliminarily joined by welding, and similarly,when a pilot bore is formed in the butting portion between the secondtab member and the metal members, inner corner portions formed by themetal members and the second tab member are preliminarily joined bywelding.

In the present invention, a pull-out hole formed when the stirring pinof the preliminary joining rotating tool used in the second tab memberjoining process was withdrawn is used as the pilot bore, or the pull-outhole is expanded in diameter and used as the pilot bore. Accordingly,boring of the pilot bore can be omitted or simplified, so that theoperation time can be shortened.

In the joining method disclosed in Document 6, a rotating tool to beused for the primary welding is used for preliminary joining, so thatthe joining strength obtained by the preliminary joining may becomeexcessive. Particularly, when the metal members to be joined are thickin thickness, the rotating tool to be used for the primary weldingbecomes large in size, when a rotating tool is used for the preliminaryjoining, not only does the rotating tool wear faster, but also a greatload is placed on the friction stir welding machine in spite ofpreparatory joining for the primary welding. Further, in Document 6, thefeed rate (movement speed) of the rotating tool in the case ofpreliminary joining must be set equal to the feed rate in the primarywelding, so that the time required for the preliminary joining cannot beshortened.

To solve this problem, the joining method of claim 32 is preferablyadopted. The joining method of claim 32 includes a preliminary joiningprocess in which a butting portion between metal members is frictionstir welded; and a primary welding process in which the butting portionbetween the preliminarily joined metal members is primarily welded,wherein the preliminary joining process is executed by using apreliminary joining rotating tool smaller than a primary weldingrotating tool to be used in the primary welding process.

Accordingly, preliminary joining can be performed with a load smallerthan in the primary welding, so that the load on the friction stirwelding machine at the time of preliminary joining can be reduced, andfurther, the movement speed (feed rate) of the preliminary joiningrotating tool can be higher than the movement speed of the primarywelding rotating tool, so that the operation time and cost for thepreliminary joining can be reduced.

It is allowed that friction stir welding is performed from the surfaceside of the metal members in the preliminary joining process, andfriction stir welding is performed from the back surface side of themetal members in the primary welding process, however, it is alsoallowed that friction stir welding is performed from the surface side ofthe metal members in both of the preliminary joining process and theprimary welding process.

In the preliminary joining process of claim 32, friction stir weldingmay be applied intermittently to the butting portion, or friction stirwelding may be performed continuously across the entire length of thebutting portion.

In the case where water cooling is applied when performing the primarywelding process, in the preliminary joining process, friction stirwelding is performed continuously across the entire length of thebutting portion between the metal members from the surface side of themetal members, and in the primary welding process, friction stir weldingis applied to the preliminarily joined butting portion from the surfaceside of the metal members. When the preliminary joining is performedintermittently, cooling water may enter between the metal members andform an oxide film on the joined surface, however, by performing thepreliminary joining continuously, cooling water hardly enters betweenthe metal members, so that the quality of the joined portion can beimproved.

In the present invention, a pilot bore is formed at the start positionof the friction stir welding in the primary welding process, and thefriction stir welding is started from the pilot bore. In other words, apilot bore is formed at a position to which the stirring pin of theprimary welding rotating tool will be inserted. Accordingly, the load(pressure) of insertion of the stirring pin of the primary weldingrotating tool can be reduced, so that the load on the friction stirwelding machine can be reduced, and further, the insertion speed of theprimary welding rotating tool can be increased, so that the operationtime required for the primary welding can be shortened.

When the pilot bore is formed, the maximum bore diameter of the pilotbore is set to be smaller than the maximum outer diameter of thestirring pin of the primary welding rotating tool, and the stirring pinis press-fitted into the pilot bore while being rotated. Accordingly,the stirring pin comes into contact with the bore wall of the pilot boreand generates frictional heat before the shoulder of the primary weldingrotating tool comes into contact with the surfaces of the metal members,etc., so that the time until plasticization and fluidization can beshortened.

In the present invention, a pull-out hole formed when the stirring pinof the preliminary joining rotating tool was withdrawn is used as thepilot bore, or the pull-out hole is expanded in diameter and used as thepilot bore. Accordingly, boring of the pilot bore can be omitted orsimplified, so that the operation time can be shortened.

When the primary welding rotating tool is rotated clockwise, atunnel-like defect, etc., may occur on the left in the travelingdirection of the primary welding rotating tool, and when the rotatingtool is rotated counterclockwise, a tunnel-like defect may occur on theright in the traveling direction, and to divide such a tunnel-likedefect, a repairing process in which a plasticized region formed throughthe primary welding process is friction stir welded, may be executed. Inthe repairing process, when the primary welding rotating tool is rotatedclockwise, the plasticized region positioned on the left in thetraveling direction of the primary welding rotating tool is frictionstir welded, and when the primary welding rotating tool is rotatedcounterclockwise, the plasticized region positioned on the right in thetraveling direction of the primary welding rotating tool is frictionstir welded.

In the repairing process, friction stir welding is performedcontinuously along the friction stir welding route of the primarywelding process. Accordingly, even when a tunnel-like defect is formedcontinuously along the friction stir welding route of the primarywelding process, it can be reliably divided, so that joining defectshardly occur.

When executing the repairing process, a repairing rotating tool smallerthan the primary welding rotating tool is used. Accordingly, theplasticized region can be prevented from excessively spreading.

When a start position or an end position of friction stir welding isprovided on a tab member disposed lateral to a butting portion betweenmetal members to be joined, along with the movement of the rotatingtool, an oxide film on the contact surface between the metal members andthe tab member is involved in the plasticized region formed on the metalmembers, so that joining defects may occur.

To solve this problem, the joining method of claim 42 is preferablyadopted. The joining method of claim 42 includes: a primary weldingprocess in which a butting portion between metal members is frictionstir welded; and a repairing process in which a plasticized regionformed on the metal members through the primary welding process isfriction stir welded, wherein in the primary welding process, a startposition or an end position of friction stir welding is provided on atab member disposed lateral to the butting portion between the metalmembers, and in the repairing process, at least the plasticized regionadjacent to the tab member is friction stir welded.

Accordingly, even when the oxide film on the contact surface between themetal members and the tab member is involved in the plasticized regionformed on the metal members, the oxide film can be divided by frictionstir welding in the repairing process, so that joining defects hardlyoccur.

In the primary welding process, when the start position of friction stirwelding is provided on the tab member disposed lateral to the buttingportion between the metal members, in the repairing process, theplasticized region on the left in the traveling direction of the primarywelding rotating tool is friction stir welded when the primary weldingrotating tool is rotated clockwise, and the plasticized region on theright in the traveling direction of the primary welding rotating tool isfriction stir welded when the primary welding rotating tool is rotatedcounterclockwise.

In the primary welding process, when an end position of friction stirwelding is provided on the tab member disposed lateral to the buttingportion between the metal members, in the repairing process, theplasticized region on the right in the traveling direction of theprimary welding rotating tool is friction stir welded when the primarywelding rotating tool is rotated clockwise, and the plasticized regionon the left in the traveling direction of the primary welding rotatingtool is friction stir welded when the primary welding rotating tool isrotated counterclockwise.

In the present invention, preferably, the repairing process is executedby using a repairing rotating tool smaller than the primary weldingrotating tool. Accordingly, the plasticized region can be prevented fromexcessively spreading.

The friction stir welding route disclosed in Document 2 is set forfriction stir welding the entire boundary surface between the bottomsurface of the depression portion formed on the butting portion and theconnecting plate without gaps, and is set not only for the purpose ofdividing a tunnel-like defect. When the friction stir welding isperformed mainly for dividing a tunnel-like defect, when the frictionstir welding route as disclosed in Document 2 is set, not only does themovement distance of the rotating tool become excessively long, but thenumber of switches of the direction of the rotating tool also increases.In other words, when the friction stir welding is performed only fordividing a tunnel-like defect, if the friction stir welding route ofDocument 2 is diverted as it is, the rotating tool is moved wastefully,and the yield may be deteriorated.

To solve the above-described problem, the joining method of claim 46 ispreferably adopted. The joining method of claim 46 includes: a primarywelding process in which a butting portion between metal members isfriction stir welded; and a traverse repairing process in which aplasticized region formed on the metal members is friction stir weldedby moving a rotating tool so that the rotating tool traverses theplasticized region a plurality of times, wherein a friction stir weldingroute in the traverse repairing process is set so that a plurality ofre-plasticized regions formed in the plasticized region through thetraverse repairing process are spaced from each other on the frictionstir welding route in the primary welding process.

According to the present invention, friction stir welding is performedso as to traverse the plasticized region formed through the primarywelding process, so that even when a tunnel-like defect is formed alongthe plasticized region, this tunnel-like defect can be divided. As inthe present invention, by spacing the plurality of re-plasticizedregions on the friction stir welding route of the primary weldingprocess from each other, in comparison with the case where there-plasticized regions are formed in the entire plasticized regionformed through the primary welding process, the number of traverses andthe number of switches of the direction of the rotating tool becomesmaller, and as a result, the total extension of the friction stirwelding route in the traverse repairing process becomes shorter, so thatwasteful movements of the rotating tool are prevented, and eventually,the tunnel-like defect can be efficiently divided.

In the present invention, preferably, re-plasticized regions adjacent toeach other are spaced from each other by a distance not less than thewidth of the re-plasticized regions. Accordingly, wasteful movements ofthe rotating tool are further prevented, and a tunnel-like defect can bemore efficiently divided.

When, in the friction stir welding route in the traverse repairingprocess, a plurality of traverse routes traversing the plasticizedregion and transfer routes connecting end portions on the same side ofthe traverse routes adjacent to each other are provided, preferably, thetransfer routes are made parallel to the friction stir welding route ofthe primary welding process. Accordingly, in comparison with the casewhere the transfer routes are inclined with respect to the friction stirwelding route of the primary welding process, the transfer routedistance becomes shorter, so that wasteful movements of the rotatingtool are further prevented, and a tunnel-like defect can be moreefficiently divided.

When the transfer routes are made parallel to the friction stir weldingroute of the primary welding process, preferably, the position of thetransfer route is set so that a plasticized region formed by moving therotating tool along the transfer route comes into contact with thelateral edge of the plasticized region formed through the primarywelding process. Accordingly, the lengths of the traverse routes can beset to the extent that the outer diameter of the shoulder of therotating tool is added to the width of the plasticized region formedthrough the primary welding process, so that wasteful movements of therotating tool are further prevented, and a tunnel-like defect can bemore efficiently divided.

In the present invention, it is desirable that the traverse repairingprocess is executed by using a rotating tool smaller than the rotatingtool to be used in the primary welding process. Accordingly, plasticizedregions (re-plasticized regions) formed through the traverse repairingprocess can be prevented from excessively spreading.

Joining defects due to the rotating tool used in the primary weldingprocess are mainly formed in the range to ⅓ from the upper end of thestirring pin, so that the length of a stirring pin of a rotating tool tobe used in the traverse repairing process is preferably set to not lessthan ⅓ of the length of the stirring pin of the rotating tool used inthe primary welding process, and if it is longer than ½, the plasticizedregion may excessively spread, so that the length of the stirring pin ispreferably set to not more than ½.

In the friction stir welding method of Document 9, the stirring pin isformed to be columnar, and the maximum bore diameter of the pilot boreis set to be smaller than the outer diameter of the stirring pin,however, in this case, the lower end surface of the stirring pin comesinto contact with the surfaces of the metal members, and this causes ahigh press-fitting resistance at the press-fitting initial stage.

To solve this problem, the friction stir welding method of claim 52 ispreferably adopted. The friction stir welding method of claim 52 inwhich a pilot bore is formed at a start position of friction stirwelding, and friction stir welding is started from the pilot bore,wherein a stirring pin of a rotating tool is formed into a circulartruncated cone shape, a maximum bore diameter of the pilot bore is setto be larger than a minimum outer diameter of the stirring pin, and themaximum bore diameter of the pilot bore is set to be smaller than amaximum outer diameter of the stirring pin, and the stirring pin ispress-fitted into the pilot bore while being rotated.

According to this friction stir welding method, a tip end portion of thestirring pin in a circular truncated cone shape enters the pilot bore,and a peripheral surface (lateral side) of the stirring pin entering thepilot bore comes into contact with a bore wall of the pilot bore, sothat the metal is plasticized and fluidized from the bore wall side. Inthis state, the stirring pin is press-fitted while the peripheralsurface of the stirring pin pushes away the plasticized and fluidizedmetal, so that a press-fitting resistance at the press-fitting initialstage can be reduced.

When the maximum bore diameter of the pilot bore is less than 50% of themaximum outer diameter of the stirring pin, the reduction degree of thepress-fitting resistance may lower, and if the maximum bore diameter ofthe pilot bore is more than 90% of the maximum outer diameter of thestirring pin, the frictional heat production by the stirring pin isreduced and the plasticized and fluidized region becomes smaller, andthe heat input is reduced, so that the load when the rotating tool movesincreases, and defects easily occur. Therefore, in the presentinvention, preferably, the maximum bore diameter of the pilot bore isset to 50% to 90% of the maximum outer diameter of the stirring pin.

If the depth of the pilot bore is excessively great, the time for boringthe pilot bore becomes long, so that preferably, the depth of the pilotbore is set to be smaller than the length of the stirring pin, however,if the depth of the pilot bore is less than 70% of the length of thestirring pin, the reduction degree of the press-fitting resistance maylower, and if the depth of the pilot bore is more than 90% of the lengthof the stirring pin, the frictional heat production by the stirring pinis reduced and the region plasticized and fluidized becomes smaller, andthe heat input is reduced, so that the load when the rotating tool movesincreases and defects easily occur. Therefore, in the present invention,preferably, the depth of the pilot bore is set to 70% to 90% of thelength of the stirring pin.

If the capacity of the pilot bore is excessively high, the plasticizedand fluidized region may become smaller and the press-fitting resistancewhen the stirring pin is press-fitted may increase, so that the capacityof the pilot bore is preferably set to be smaller than the volume of thestirring pin, however, if the capacity of the pilot bore is less than40% of the volume of the stirring pin, the reduction degree of thepress-fitting resistance may lower, and if the capacity of the pilotbore is more than 80% of the volume of the stirring pin, the frictionalheat production by the stirring pin is reduced and the plasticized andfluidized region becomes smaller, and the heat input is reduced, so thatthe load when the rotating tool moves increases and defects easilyoccur. Therefore, in the present invention, the capacity of the pilotbore is preferably set to 40% to 80% of the volume of the stirring pin.

When the stirring pin is inserted into the start position of frictionstir welding, the metal in a normal temperature state must beplasticized and fluidized, so that the insertion operation takes time.

To solve this problem, the friction stir welding method of claim 58 ispreferably adopted. According to the friction stir welding method ofclaim 58, the rotating speed of the rotating tool when the stirring pinof the rotating tool is inserted into a start position of friction stirwelding is set to be higher than the rotating speed of the rotating toolwhen the rotating tool is moved toward an end position of friction stirwelding from the start position.

Accordingly, in comparison with the case where the rotating speed of therotating tool when the stirring pin is inserted into the start positionof friction stir welding (rotating speed for insertion) is set to beequal to the rotating speed of the rotating tool when the rotating toolis moved from the start position to the end position of friction stirwelding (rotating speed for movement), the time until the metal isplasticized and fluidized becomes shorter, so that the stirring pininserting operation at the start position of friction stir welding canbe quickly performed.

If the rotating speed for insertion becomes greater than 3.0 times therotating speed for movement, the heat input into the metal increases andthe metal temperature excessively rises, so that the rotating speed forinsertion is preferably not more than 3.0 times. If the rotating speedfor insertion is less than 1.5 times the rotating speed for movement ofthe rotating tool, the operation time shortening effect becomes smaller,so that the rotating speed for insertion is preferably not less than 1.5times.

When the stirring pin is withdrawn from the end position of frictionstir welding, the plasticized and fluidized metal clings to the stirringpin, so that the stirring pin withdrawal resistance (pulling resistance)is great, and the stirring pin withdrawing operation takes time.

To solve this problem, the friction stir welding method of claim 60 ispreferably adopted. According to the friction stir welding of claim 60,the rotating speed of the rotating tool when the stirring pin of therotating tool reaching the end position of friction stir welding iswithdrawn from the end position is set to be higher than the rotatingspeed of the rotating tool when the rotating tool is moved from thestart position to the end position of friction stir welding.

Accordingly, in comparison with the case where the rotating speed of therotating tool when the stirring pin is withdrawn from the end positionof friction stir welding (rotating speed for withdrawal) is equal to therotating speed for movement, the withdrawal resistance of the stirringpin becomes smaller, so that the stirring pin withdrawing operation atthe end position of friction stir welding can be quickly performed.

If the rotating speed for withdrawal becomes more than 3.0 times therotating speed for movement of the rotating tool, the heat input intothe metal increases and the temperature of the metal excessively rises,so that the rotating speed for withdrawal is preferably not more than3.0 times. If the rotating speed for withdrawal is less than 1.5 timesthe rotating speed for movement, the operation time shortening effectbecomes smaller, so that the rotating speed for withdrawal is preferablynot less than 1.5 times.

Effects of the Invention

According to the joining method of the present invention, airtightnessand water-tightness of the joined portion can be improved. Further,according to the joining method and the friction stir welding method ofthe present invention, joining operations can be improved in efficiencyand speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) to FIG. 1( d) are drawings for describing arrangement ofmetal members, a first tab member, and a second tab member of a firstembodiment, and FIG. 1( a) is a perspective view, FIG. 1( b) is a planview, FIG. 1( c) is a sectional view along the I-I line of FIG. 1( b),and FIG. 1( d) is a sectional view along the II-II line of FIG. 1( b);

FIG. 2( a) is a side view for describing a preliminary joining rotatingtool, and FIG. 2( b) is a side view for describing a primary weldingrotating tool;

FIG. 3( a) and FIG. 3( b) are schematic side views for describing asituation of insertion of the preliminary joining rotating tool to astart position;

FIG. 4 is a plan view for describing a first tab member joining process,a preliminary joining process, and a second tab member joining processof the first embodiment;

FIG. 5( a) is a sectional view along the III-III line of FIG. 4, andFIG. 5( b) and FIG. 5( c) are sectional views for describing a firstprimary welding process of the first embodiment;

FIG. 6( a) and FIG. 6( b) are views for describing a region to befriction stir welded in a first repairing process of the firstembodiment, and FIG. 6( a) is a plan view and FIG. 6( b) is a sectionalview along IV-IV line of FIG. 6( a);

FIG. 7 is a plan view for describing the first repairing process of thefirst embodiment;

FIG. 8 is a plan view for describing a first traverse repairing processof the first embodiment;

FIG. 9 is a sectional view along V-V line of FIG. 8;

FIG. 10( a) to FIG. 10( c) are sectional views for describing a secondprimary welding process of the first embodiment;

FIG. 11( a) is a side view showing a primary welding rotating tool to beused in the first primary welding process, and FIG. 11( b) is a sideview showing a primary welding rotating tool to be used in the secondprimary welding process;

FIG. 12 is a plan view for describing a modified example of the firsttab member joining process and the second tab member joining process ofthe first embodiment;

FIG. 13 is a plan view for describing a modified example of therepairing process of the first embodiment;

FIG. 14 is a plan view for describing a modified example of the traverserepairing process of the first embodiment;

FIG. 15( a) and FIG. 15( b) are views for describing arrangement ofmetal members, a first tab member, and a second tab member of a secondembodiment, and FIG. 15( a) is a perspective view and FIG. 15( b) is aplan view;

FIG. 16( a) and FIG. 16( b) are views for describing a first preparatoryprocess of the second embodiment;

FIG. 17 is a plan view for describing a modified example of the firstpreparatory process of the second embodiment;

FIG. 18 is a plan view for describing a first primary welding process ofthe second embodiment;

FIG. 19( a) and FIG. 19( b) are plan views for describing a modifiedexample of the first primary welding process of the second embodiment;

FIG. 20( a) and FIG. 20( b) are plan views for describing a firstrepairing process of the second embodiment;

FIG. 21 is a plan view for describing a first traverse repairing processof the second embodiment;

FIG. 22 is a sectional view for describing a second primary weldingprocess of the second embodiment; and

FIG. 23 is a plan view for describing a modified example of a first tabmember joining process and a second tab member joining process of thesecond embodiment.

DESCRIPTION OF THE REFERENCE NUMERALS

-   1 metal member-   2 first tab member-   3 second tab member-   J1 to J3 butting portion-   A preliminary joining rotating tool-   A1 shoulder-   A2 stirring pin-   B primary welding rotating tool-   B1 shoulder-   B2 stirring pin-   C repairing rotating tool-   D traversing rotating tool-   P1 pilot bore-   W1, W2 plasticized region-   W3 re-plasticized region

BEST MODE FOR CARRYING OUT THE INVENTION

As a best mode for carrying out the present invention, a method forjoining metal members to each other by using friction stir welding, inwhich after a butting portion between metal members is subjected tofriction stir welding as preliminary joining, the preliminarily joinedbutting portion is subjected to friction stir welding as primarywelding, is illustrated.

First Embodiment

In a first embodiment, as shown in FIG. 1( a) to FIG. 1( d), linearjoining of metal members 1, 1 is illustrated.

First, metal members 1, 1 to be joined will be described in detail, anda first tab member 2 and a second tab member 3 to be used when joiningthese metal members 1, 1 will also be described in detail.

The metal member 1 is made of metal materials which can be friction stirwelded such as aluminum, an aluminum alloy, copper, a copper alloy,titanium, a titanium alloy, magnesium, and a magnesium alloy, etc. Inthe present invention, one metal member 1 and the other metal member 1are made of metal materials having the same composition. The shapes anddimensions of the metal members 1, 1 are not specifically limited,however, preferably, at least, their thicknesses at the butting portionJ1 are equal.

A first tab member 2 and a second tab member 3 are disposed so as tosandwich the butting portion J1 between the metal members 1, 1, and aredisposed along the metal members 1, 1 and cover seams (boundaries)between the metal members 1, 1 on the lateral sides 14 of the metalmembers 1. The materials of the first tab member 2 and the second tabmember 3 are not specifically limited, however, in the presentembodiment, they are made of metal materials having the same compositionas that of the metal members 1. The shapes and dimensions of the firsttab member 2 and the second tab member 3 are not specifically limited,however, in the present invention, their thicknesses are set to be equalto the thickness of the metal members 1 at the butting portion J1.

Next, a rotating tool A to be used for preliminary joining (hereinafter,referred to as “preliminary joining rotating tool A”) and a rotatingtool B to be used for primary welding (hereinafter, referred to as“primary welding rotating tool B”) will be described in detail withreference to FIG. 2( a) and FIG. 2( b).

The preliminary joining rotating tool A shown in FIG. 2( a) is made of ametal material such as tool steel harder than the metal members 1, andincludes a shoulder A1 in a columnar shape and a stirring pin (probe) A2provided so as to project from the lower end surface A11 of the shoulderA1. The dimensions and shape of the preliminary joining rotating tool Aare set according to the materials and thicknesses of the metal members1, and at least, they are made smaller than the primary welding rotatingtool B (see FIG. 2( b)) to be used in a first primary welding processdescribed later. Accordingly, preliminary joining can be performed witha load smaller than in the primary welding, so that the load on afriction stir welding machine during preliminary joining can be reduced,and further, the movement speed (feed rate) of the preliminary joiningrotating tool A can be set to be higher than that of the primary weldingrotating tool B, so that the operation time and cost for the preliminaryjoining can be reduced.

The lower end surface A11 of the shoulder A1 is a portion which performsa role of preventing plasticized and fluidized metal from scatteringaround, and is shaped into a depressed surface in the presentembodiment. The outer diameter X₁ of the shoulder A1 is not specificallylimited, however, in the present embodiment, it is smaller than theouter diameter Y₁ of the shoulder B1 of the primary welding rotatingtool B.

The stirring pin A2 is hung down from the center of the lower endsurface A11 of the shoulder A1, and is shaped into a circular truncatedcone shape in the present embodiment. On the peripheral surface of thestirring pin A2, a stir wing carved spirally is formed. The outerdiameter of the stirring pin A2 is not specifically limited, however, inthe present embodiment, the maximum outer diameter (upper end diameter)X₂ is smaller than the maximum outer diameter (upper end diameter) Y₂ ofthe stirring pin B2 of the primary welding rotating tool B, and theminimum outer diameter (lower end diameter) X₃ is smaller than theminimum outer diameter (lower end diameter) Y₃ of the stirring pin B2.The length L_(A) of the stirring pin A2 is preferably set to 3% to 15%of the thickness t (see FIG. 2( b)) of the metal members 1 at thebutting portion J1 (see FIG. 1( a)), and at least, preferably, it is setto be smaller than the length L₁ of the stirring pin B2 of the primarywelding rotating tool B.

The primary welding rotating tool B shown in FIG. 2( b) is made of ametal material such as tool steel harder than the metal members 1, andincludes a shoulder B1 in a columnar shape and a stirring pin (probe) B2provided so as to project from the lower end surface B11 of the shoulderB1.

The lower end surface B11 of the shoulder B1 is shaped into a depressedsurface as in the case of the preliminary joining rotating tool A. Thestirring pin B2 is hung down from the center of the lower end surfaceB11 of the shoulder B1 and is shaped into a circular truncated coneshape toward the end in the present embodiment. On the peripheralsurface of the stirring pin B2, a stir wing carved spirally is formed.The length L₁ of the stirring pin B2 is preferably set to not less than½ and not more than ¾ of the thickness t of the metal members 1 at thebutting portion J1 (see FIG. 1( a)), and more preferably set so as tosatisfy the relation of 1.01≦2L₁/t≦1.10.

Hereinafter, a joining method of the present embodiment will bedescribed in detail. The joining method of the present embodimentincludes: (1) a preparation process, (2) a first preparatory process,(3) a first primary welding process, (4) a first repairing process, (5)a first traverse repairing process, (6) a second preparatory process,(7) a second primary welding process, (8) a second repairing process,and (9) a second traverse repairing process. The first preparatoryprocess, the first primary welding process, the first repairing process,and the first traverse repairing process are executed from the surface12 side of the metal members 1, and the second preparatory process, thesecond primary welding process, the second repairing process, and thesecond traverse repairing process are executed from the back surface 13side of the metal members 1.

(1) Preparation Process

The preparation process will be described with reference to FIG. 1( a)to FIG. 1( d). The preparation process is for preparing metal members 1,1 to be joined and splint members (the first tab member 2 and the secondtab member 3) on which a start position and an end position of frictionstir welding are provided, and in the present embodiment, includes abutting process in which the metal members 1, 1 to be joined are buttedagainst each other, a tab member disposing process in which the firsttab member 2 and the second tab member 3 are disposed on both sides ofthe butting portion J1 between the metal members 1, 1, and a weldingprocess in which the first tab member 2 and the second tab member 3 arepreliminarily joined to the metal members 1, 1 by welding.

In the butting process, as shown in FIG. 1( c), with the lateral side 11of one metal member 1, the lateral side 11 of the other metal member 1is brought into contact, and the surface 12 of one metal member 1 andthe surface 12 of the other metal member 1 are made flush with eachother, and further, the back surface 13 of one metal member 1 and theback surface 13 of the other metal member 1 are made flush with eachother.

In the tab member disposing process, as shown in FIG. 1( b), the firsttab member 2 is disposed on one end side of the butting portion J1between the metal members 1, 1 and its contact surface 21 is broughtinto contact with the lateral sides 14, 14 of the metal members 1, 1,and the second tab member 3 is disposed on the other end side of thebutting portion J1 and its contact surface 31 is brought into contactwith the lateral sides 14, 14 of the metal members 1, 1. At this time,as shown in FIG. 1( d), the surface 22 of the first tab member 2 and thesurface 32 of the second tab 3 are made flush with the surfaces 12 ofthe metal members 1, and the back surface 23 of the first tab member 2and the back surface 33 of the second tab member 3 are made flush withthe back surfaces 13 of the metal members 1.

In the welding process, as shown in FIG. 1( a) and FIG. 1( b), innercorner portions 2 a, 2 a formed by the metal members 1 and the first tabmember 2 (that is, the corner portions 2 a, 2 a formed by the lateralsides 14 of the metal members 1 and the lateral sides 24 of the firsttab member 2) are welded to join the metal members 1 and the first tabmember 2, and inner corner portions 3 a, 3 a formed by the metal members1 and the second tab member 3 (that is, corner portions 3 a, 3 a formedby the lateral sides 14 of the metal members 1 and the lateral sides 34of the second tab member 3) are welded to join the metal members 1 andthe second tab member 3. The entire lengths of the inner corner portions2 a and 3 a may be continuously welded, or may be intermittently welded.

After the preparation process is finished, the metal members 1, 1, thefirst tab member 2, and the second tab member 3 are placed on a platformof a friction stir welding machine not shown, and locked so as not tomove by using a jig such as a clamp not shown. When the welding processis omitted, the butting process and the tab member disposing process areexecuted on the platform of the friction stir welding machine not shown.

(2) First Preparatory Process

The first preparatory process is performed previous to the first primarywelding process, and in the present embodiment, includes a first tabmember joining process in which the butting portion J2 between the metalmembers 1, 1 and the first tab member 2 is joined, a preliminary joiningprocess in which the butting portion J1 between the metal members 1, 1is preliminarily joined, a second tab member joining process in which abutting portion J3 between the metal members 1, 1 and the second tabmember 3 is joined, and a pilot bore forming process in which a pilotbore is formed at a start position of friction stir welding of the firstprimary welding process.

In the first preparatory process of the present embodiment, as shown inFIG. 4, by moving one preliminary joining rotating tool A to form aone-stroke movement locus (bead), the butting portions J1, J2, and J3are successively friction stir welded. In other words, the stirring pinA2 (see FIG. 2( a)) of the preliminary joining rotating tool A insertedinto the start position S_(P) of friction stir welding is moved to theend position E_(P) without being withdrawn midway, whereby the first tabmember joining process, the preliminary joining process, and the secondtab member joining process are successively executed. In the presentembodiment, the start position Sp of friction stir welding is providedon the first tab member 2, and the end position E_(P) is provided on thesecond tab member 3, however, this is not intended to limit thepositions of the start position S_(P) and the end position E_(P).

Friction stir welding steps in the first preparatory process of thepresent embodiment will be described with reference to FIG. 3( a) toFIG. 3( b) and FIG. 4.

First, as shown in FIG. 3( a), the preliminary joining rotating tool Ais positioned just above the start position S_(P) provided on anappropriate location on the first tab member 2, and then, thepreliminary joining rotating tool A is moved down while being rotatedclockwise and the stirring pin A2 is pressed against the start positionS_(P). The rotating speed of the preliminary joining rotating tool A isset according to the dimensions and shape of the stirring pin A2 and thematerials and the thicknesses of the metal members 1 to be friction stirwelded, etc., and in many cases, the rotating speed is set within arange of 500 to 2000 (rpm).

When the stirring pin A2 comes into contact with the surface 22 of thefirst tab member 2, the metal around the stirring pin A2 is plasticizedand fluidized due to frictional heat, and as shown in FIG. 3( b), thestirring pin A2 is inserted into the first tab member 2. The insertionspeed (moving-down speed) of the preliminary joining rotating tool A isset according to the dimensions and shape of the stirring pin A2 and thematerial and thickness etc., of the member on which the start positionS_(P) is provided, and in many cases, the insertion speed is set withina range of 30 to 60 (mm/min).

When the entirety of the stirring pin A2 gets into the first tab member2 and the entire lower end surface A11 of the shoulder A1 comes intocontact with the surface 22 of the first tab member 2, as shown in FIG.4, the preliminary joining rotating tool A is relatively moved to thestart point s2 of the first tab member joining process while beingrotated.

The movement speed (feed rate) of the preliminary joining rotating toolA is set according to the dimensions and shape of the stirring pin A2and the materials and thicknesses of the metal members 1 to be frictionstir welded, etc., and in many cases, the movement speed is set within arange of 100 to 1000 (mm/min). The rotating speed of the preliminaryjoining rotating tool A for the traveling is set to be equal to or lowerthan the rotating speed for the inserting. When the preliminary joiningrotating tool A is moved, the axis line of the shoulder A1 may beinclined slightly to the back side of the traveling direction withrespect to the vertical line, however, when it is not inclined but setvertically, the traveling direction of the preliminary joining rotatingtool A can be easily changed, and complicated movements thereof arepossible. When the preliminary joining rotating tool A is moved, themetal around the stirring pin A2 is sequentially plasticized andfluidized, and at a position apart from the stirring pin A2, theplasticized and fluidized metal hardens again.

When friction stir welding is performed continuously to the start points2 of the first tab member joining process by relatively moving thepreliminary joining rotating tool A, the preliminary joining rotatingtool A is not withdrawn at the start point s2 but directly shifted tothe first tab member joining process.

In the first tab member joining process, the butting portion J2 betweenthe first tab member 2 and the metal members 1, 1 is friction stirwelded. In detail, a friction stir welding route is set on the seam(boundary) between the metal members 1, 1 and the first tab member 2,and by relatively moving the preliminary joining rotating tool A alongthis route, the butting portion J2 is friction stir welded. In thepresent embodiment, friction stir welding is performed continuously fromthe start point s2 to the end point e2 of the first tab member joiningprocess without withdrawing the preliminary joining rotating tool Amidway.

When the preliminary joining rotating tool A is rotated clockwise, finejoining defects may occur on the left in the traveling direction of thepreliminary joining rotating tool A, and therefore preferably, thepositions of the start point s2 and the end point e2 of the first tabmember joining process are set so that the metal members 1, 1 arepositioned on the right in the traveling direction of the preliminaryjoining rotating tool A. Accordingly, joining defects hardly occur onthe metal member 1 side, so that a high-quality joined body can beobtained.

Incidentally, when the preliminary joining rotating tool A is rotatedcounterclockwise, fine joining defects may occur on the right in thetraveling direction of the preliminary joining rotating tool A, andtherefore preferably, the positions of the start point and end point ofthe first tab member joining process are set so that the metal members1, 1 are positioned on the left in the traveling direction of thepreliminary joining rotating tool A. In detail, the start point isprovided at the position of the end point e2 set in the case where thepreliminary joining rotating tool A is rotated clockwise, and the endpoint is provided at the position of the start point s2 set in the casewhere the preliminary joining rotating tool A is rotated clockwisealthough these are not shown.

When the stirring pin A2 of the preliminary joining rotating tool A getsinto the butting portion J2, a force to separate the metal members 1 andthe first tab member 2 from each other acts, however, the inner cornerportions 2 a formed by the metal members 1 and the first tab member 2are preliminarily joined by welding, so that separation does not occurbetween the metal members 1 and the first tab member 2.

When the preliminary joining rotating tool A reaches the endpoint e2 ofthe first tab member joining process, friction stir welding is notfinished at the endpoint e2 but continued to the start point s1 of thepreliminary joining process, and directly shifted to the preliminaryjoining process. In other words, friction stir welding is continued fromthe end point e2 of the first tab member joining process to the startpoint s1 of the preliminary joining process without withdrawing thepreliminary joining rotating tool A, and further, the process is shiftedto the preliminary joining process without withdrawing the preliminaryjoining rotating tool A at the start point s1. Accordingly, theoperation for withdrawing the preliminary joining rotating tool A at theendpoint e2 of the first tab member joining process becomes unnecessary,and the operation for inserting the preliminary joining rotating tool Aat the start point s1 of the preliminary joining process becomesunnecessary, so that the preparatory joining operation can be improvedin efficiency and speed.

In the present embodiment, a friction stir welding route from the endpoint e2 of the first tab member joining process to the start point s1of the preliminary joining process is set on the first tab member 2, anda movement locus of the traveling of the preliminary joining rotatingtool A from the end point e2 of the first tab member joining process tothe start point s1 of the preliminary joining process is formed on thefirst tab member 2. Accordingly, in the process from the end point e2 ofthe first tab member joining process to the start point s1 of thepreliminary joining process, joining defects hardly occur in the metalmembers 1, 1, so that a high-quality joined body can be obtained.

A spacing distance d₂ between the friction stir welding route in thefirst tab member joining process and a portion parallel to the frictionstir welding route in the first tab member joining process of thefriction stir welding route from the end point e2 of the first tabmember joining process to the start point s1 of the preliminary joiningprocess is secured not less than the outer diameter X₁ (see FIG. 2( a))of the shoulder A1 of the preliminary joining rotating tool A. In otherwords, the spacing distance d₂ between the movement locus formed whenthe preliminary joining rotating tool A is moved from the start point s2to the end point e2 of the first tab member joining process and themovement locus formed when the preliminary joining rotating tool A ismoved from the end point e2 of the first tab member joining process tothe start point s1 of the preliminary joining process is secured notless than the outer diameter X₁ of the shoulder A1 of the preliminaryjoining rotating tool A. Accordingly, in the process from the end pointe2 of the first tab member joining process to the start point s1 of thepreliminary joining process, even if joining defects occur on the metalmember 1 side of the preliminary joining rotating tool A, the joiningdefects hardly reach the metal members 1, so that a high-quality joinedbody can be obtained.

In the preliminary joining process, the butting portion J1 (see FIG. 1(a)) of the metal members 1, 1 is friction stir welded. In detail, afriction stir welding route is set on the seam (boundary) between themetal members 1, 1, and by relatively moving the preliminary joiningrotating tool A along this route, friction stir welding is performedcontinuously across the entire length of the butting portion J1. In thepresent embodiment, the friction stir welding is continuously performedfrom the start point s1 to the end point e1 of the preliminary joiningprocess without withdrawing the preliminary joining rotating tool Amidway. Accordingly, the withdrawing operation of the preliminaryjoining rotating tool A during the preliminary joining process is nolonger necessary, so that the preparatory joining operation can befurther improved in efficiency and speed.

When the preliminary joining rotating tool A reaches the end point e1 ofthe preliminary joining process, the friction stir welding is notfinished at the endpoint e1 but continued to the start point s3 of thesecond tab member joining process, and is directly shifted to the secondtab member joining process. In other words, the friction stir welding iscontinued from the end point e1 of the preliminary joining process tothe start point s3 of the second tab member joining process withoutwithdrawing the preliminary joining rotating tool A, and further,shifted to the second tab member joining process without withdrawing thepreliminary joining rotating tool A at the start point s3. Accordingly,the operation for withdrawing the preliminary joining rotating tool A atthe end point e1 of the preliminary joining process becomes unnecessary,and the operation for inserting the preliminary joining rotating tool Aat the start point s3 of the second tab member joining process becomesunnecessary, so that the preparatory joining operation can be furtherimproved in efficiency and speed.

In the present embodiment, the friction stir welding route from the endpoint e1 of the preliminary joining process to the start point s3 of thesecond tab member joining process is set on the second tab member 3, andthe movement locus of the traveling of the preliminary joining rotatingtool A from the end point e1 of the preliminary joining process to thestart point s3 of the second tab member joining process is formed on thesecond tab member 3. Accordingly, in the process from the end point e1of the preliminary joining process to the start point s3 of the secondtab member joining process, joining defects hardly occur in the metalmembers 1, so that a high-quality joined body can be obtained.

The spacing distance d₃ between a portion parallel to the friction stirwelding route of the second tab member joining process described laterin the friction stir welding route from the end point e1 of thepreliminary joining process to the start point s3 of the second tabmember joining process and the friction stir welding route of the secondtab member joining process is secured not less than the outer diameterX₁ (see FIG. 2( a)) of the shoulder A1 of the preliminary joiningrotating tool A. In other words, the spacing distance d₃ between themovement locus formed when the preliminary joining rotating tool A ismoved from the end point e1 of the preliminary joining process to thestart point s3 of the second tab member joining process and the movementlocus formed when the preliminary joining rotating tool A is moved fromthe start point s3 to the end point e3 of the second tab member joiningprocess is secured not less than the outer diameter X₁ of the shoulderA1 of the preliminary joining rotating tool A. Accordingly, in theprocess from the end point e1 of the preliminary joining process to thestart point s3 of the second tab member joining process, even whenjoining defects occur on the metal member 1 side of the preliminaryjoining rotating tool A, the joining defects hardly reach the metalmembers 1, so that a high-quality joined body can be obtained.

In the second tab member joining process, the butting portion J3 betweenthe metal members 1, 1 and the second tab member 3 is friction stirwelded. In detail, a friction stir welding route is set on the seam(boundary) between the metal members 1, 1 and the second tab member 3,and by relatively moving the preliminary joining rotating tool A alongthis route, the butting portion J3 is friction stir welded. In thepresent embodiment, friction stir welding is continued from the startpoint s3 to the end point e3 of the second tab member joining processwithout withdrawing the preliminary joining rotating tool A midway.

The preliminary joining rotating tool A is rotated clockwise, therefore,the positions of the start point s3 and the endpoint e3 of the secondtab member joining process are set so that the metal members 1, 1 arepositioned on the right in the traveling direction of the preliminaryjoining rotating tool A. Accordingly, joining defects hardly occur onthe metal member 1 side, so that a high-quality joined body can beobtained. Incidentally, when the preliminary joining rotating tool A isrotated counterclockwise, preferably, the positions of the start pointand the end point of the second tab member joining process are set sothat the metal members 1, 1 are positioned on the left in the travelingdirection of the preliminary joining rotating tool A. In detail, thestart point is provided at the position of the endpoint e3 set when thepreliminary joining rotating tool A is rotated clockwise, and the endpoint is set at the position of the start point s3 set when thepreliminary joining rotating tool A is rotated clockwise although theseare not shown.

When the stirring pin A2 (see FIG. 2( a)) of the preliminary joiningrotating tool A gets into the butting portion J3, a force to separatethe metal members 1 and the second tab member 3 from each other acts,however, the inner corner portions 3 a between the metal members 1 andthe second tab member 3 are preliminarily joined by welding, so thatseparation does not occur between the metal members 1 and the second tabmember 3.

When the preliminary joining rotating tool A reaches the end point e3 ofthe second tab member joining process, friction stir welding is notfinished at the end point e3 but continued to the end position E_(P)provided on the second tab member 3. In the present embodiment, the endposition E_(P) is provided on the extension of the seam (boundary)between the metal members 1, 1 appearing on the surface 12 side of themetal members 1. Incidentally, the end position E_(P) is the startposition S_(M1) of the friction stir welding in the first primarywelding process described later.

When the preliminary joining rotating tool A reaches the end positionE_(P), the preliminary joining rotating tool A is moved up while beingrotated to withdraw the stirring pin A2 (see FIG. 2( a)) from the endposition E_(P).

The withdrawal speed (moving-up speed) of the preliminary joiningrotating tool A is set according to the dimensions and shape of thestirring pin A2 and the material and thickness, etc., of the member onwhich the end position E_(P) is provided, etc., and in many cases, thewithdrawal speed is set in the range of 30 to 60 (mm/min). The rotatingspeed of the preliminary joining rotating tool A for the withdrawing isset to be equal to or higher than the rotating speed for the traveling.

Subsequently, the pilot bore forming process is executed. The pilot boreforming process is a process for forming a pilot bore P1 at the startposition S_(M1) of friction stir welding of the first primary weldingprocess as shown in FIG. 2( b). In other words, the pilot bore formingprocess is a process for forming a pilot bore P1 at a position to whichthe stirring pin B2 of the primary welding rotating tool B will beinserted.

The pilot bore P1 is formed for reducing the insertion resistance(press-fitting resistance) of the stirring pin B2 of the primary weldingrotating tool B, and in the present embodiment, it is formed byexpanding the diameter of a pull-out hole H1 formed when the stirringpin A2 (see FIG. 2( a)) of the preliminary joining rotating tool A iswithdrawn by means of drilling not shown, etc. By using the pull-outhole H1, the process to form the pilot bore P1 can be simplified, sothat the operation time can be shortened. The form of the pilot bore P1is not specifically limited, and in the present embodiment, it is formedinto a cylindrical shape. In the present embodiment, the pilot bore P1is formed in the second tab member 3, however, the position of the pilotbore P1 is not specifically limited, and it may be formed in the firsttab member 2 or may be formed in the butting portion J2 or J3, however,preferably, it is formed on the extension of the seam (boundary) betweenthe metal members 1, 1 appearing on the surface 12 side of the metalmembers 1 as described in the present embodiment.

The maximum bore diameter Z₁ of the pilot bore P1 is smaller than themaximum outer diameter (upper end diameter) Y₂ of the stirring pin B2 ofthe primary welding rotating tool B, however, preferably, it is set to50% to 90% of the maximum outer diameter Y₂ of the stirring pin B2. Ifthe maximum bore diameter Z₁ of the pilot bore P1 is less than 50% ofthe maximum outer diameter Y₂ of the stirring pin B2, the reductiondegree of the press-fitting resistance of the stirring pin B2 may becomelower, and if the maximum bore diameter Z₁ of the pilot bore P1 is morethan 90% of the maximum outer diameter Y₂ of the stirring pin B2,frictional heat production by the stirring pin B2 is reduced and theplasticized and fluidized region becomes smaller, and the heat input isreduced, therefore, the load for moving the primary welding rotatingtool B increases and defects easily occur.

If the depth Z₂ of the pilot bore P1 is excessively great, the timerequired for machining the pilot bore P1 becomes longer, so thatpreferably, the depth Z₂ of the pilot bore P1 is set to be smaller thanthe length L₁ of the stirring pin B2, however, if the depth Z₂ of thepilot bore P1 is less than 70% of the length L₁ of the stirring pin B2,the reduction degree of the press-fitting resistance may become lower,and if the depth Z₂ of the pilot bore P1 is more than 90% of the lengthL₁ of the stirring pin B2, frictional heat production by the stirringpin B2 is reduced and the plasticized and fluidized region becomessmaller, and the heat input is reduced, therefore, the load for movingthe primary welding rotating tool B increases and defects easily occur.Accordingly, the depth Z₂ of the pilot bore P1 is preferably set to 70%to 90% of the length L₁ of the stirring pin B2.

If the capacity of the pilot bore P1 is excessively large, theplasticized and fluidized region may become smaller and thepress-fitting resistance when the stirring pin B2 is press-fitted mayincrease, therefore, preferably, the capacity of the pilot bore P1 isset to be smaller than the volume of the stirring pin B2, however, ifthe capacity of the pilot bore P1 is less than 40% of the volume of thestirring pin B2, the reduction degree of the press-fitting resistancemay become lower, and if the capacity of the pilot bore P1 is more than80% of the volume of the stirring pin B2, the frictional heat productionby the stirring pin B2 is reduced and the plasticized and fluidizedregion becomes smaller, and the heat input is reduced, therefore, theload for moving the primary welding rotating tool B increases anddefects easily occur. Accordingly, the capacity of the pilot bore P1 ispreferably set to 40% to 80% of the volume of the stirring pin B2.

In the present embodiment, the case where the pilot bore P1 is formed byexpanding the diameter of the pull-out hole H1 of the stirring pin A2(see FIG. 2( a)) of the preliminary joining rotating tool A isillustrated, however, when the maximum outer diameter X₂ of the stirringpin A2 is larger than the minimum outer diameter Y₃ of the stirring pinB2 of the primary welding rotating tool B and the maximum outer diameterX₂ of the stirring pin A2 is smaller than the maximum outer diameter Y₂of the stirring pin B2 (Y₃<X₂<Y₂), the pull-out hole H1 of the stirringpin A2 can be used as the pilot bore P1 as it is.

(3) First Primary Welding Process

The first primary welding process is a process for primarily welding thebutting portion J1 between the metal members 1, 1. In the first primarywelding process of the present embodiment, by using the primary weldingrotating tool B shown in FIG. 2( b), the preliminarily joined buttingportion J1 is friction stir welded from the surface 12 side of the metalmembers 1.

In the first primary welding process, as shown in FIG. 5( a) to FIG. 5(c), the stirring pin B2 of the primary welding rotating tool B isinserted (press-fitted) into the pilot bore P1 formed at the startposition S_(M1), and the inserted stirring pin B2 is moved to the endposition E_(M1) without being withdrawn midway. In other words, in thefirst primary welding process, friction stir welding is started from thepilot bore P1, and is performed continuously to the end position E_(M1).In the present embodiment, the start position S_(M1) of friction stirwelding is provided on the second tab member 3 and the end positionE_(M1) is provided on the first tab member 2, however, this is notintended to limit the positions of the start position S_(M1) and the endposition E_(M1).

The first primary welding process will be described in detail withreference to FIG. 5( a) to FIG. 5( c).

First, as shown in FIG. 5( a), the primary welding rotating tool B ispositioned just above the pilot bore P1 (start position S_(M1)), andsubsequently, the primary welding rotating tool B is moved down whilebeing rotated clockwise to insert the tip end of the stirring pin B2into the pilot bore P1. When the stirring pin B2 is inserted into thepilot bore P1, the peripheral surface (lateral side) of the stirring pinB2 comes into contact with the bore wall of the pilot bore P1 andplasticizes and fluidizes the metal from the bore wall. In this state,the stirring pin B2 is press-fitted while pushing-away the plasticizedand fluidized metal by the peripheral surface of the stirring pin B2, sothat the press-fitting resistance at the press-fitting initial stage canbe reduced, and the stirring pin B2 comes into contact with the borewall of the pilot bore P1 and produces frictional heat before theshoulder B1 of the primary welding rotating tool B comes into contactwith the surface 32 of the second tab member 3, so that the time untilplasticization and fluidization can be shortened. In other words, theload on the friction stir welding machine can be reduced, and inaddition, the operation time required for this welding can be shortened.

The rotating speed of the primary welding rotating tool B when insertingthe stirring pin B2 of the primary welding rotating tool B to the startposition S_(M1) of friction stir welding (rotating speed for insertion)is set according to the dimensions and shape of the stirring pin B2 andthe materials and thicknesses of the metal members 1 to be friction stirwelded, etc., and in many cases, the rotating speed is set in the rangeof 70 to 700 (rpm), however, the rotating speed is preferably set to behigher than the rotating speed (rotating speed for movement) of theprimary welding rotating tool B when moving the primary welding rotatingtool B from the start position S_(M1) to the end position E_(M1) offriction stir welding. Accordingly, in comparison with the case wherethe rotating speed for insertion is set equal to the rotating speed formovement, the time required for plasticizing and fluidizing the metalbecomes shorter, so that the operation for inserting the stirring pin B2to the start position S_(M1) can be quickly performed.

If the rotating speed for insertion of the primary welding rotating toolB is higher than 3.0 times the rotating speed for movement, the heatinput into the metal increases and the temperature of the metalexcessively increases, and if the rotating speed for insertion is lowerthan 1.5 times the rotating speed for movement, the operation timeshortening effect becomes smaller, so that the rotating speed forinsertion of the primary welding rotating tool B is preferably set to1.5 times to 3.0 times as high as the rotating speed for movement.

The insertion speed (moving-down speed) of the primary welding rotatingtool B is set according to the dimensions and shape of the stirring pinB2 and the material and thickness of the member on which the startposition S_(M1) is provided, etc., and in many cases, the insertionspeed is set in the range of 5 to 60 (mm/min).

When the entire stirring pin B2 gets into the second tab member 3 andthe entire lower end surface B11 of the shoulder B1 comes into contactwith the surface 32 of the second tab member 3, as shown in FIG. 5( b),the primary welding rotating tool B is relatively moved toward one endof the butting portion J1 between the metal members 1, 1 whileperforming friction stir welding, and is further made to pass across thebutting portion J3 and enter the butting portion J1. By moving theprimary welding rotating tool B, the metal around the stirring pin B2 issuccessively plasticized and fluidized, and at a position apart from thestirring pin B2, the plasticized and fluidized metal hardens again andforms a plasticized region W1 (hereinafter, referred to as “surfaceplasticized region W1”).

The movement speed (feed rate) of the primary welding rotating tool B isset according to the dimensions and shape of the stirring pin B2 and thematerials and thicknesses of the metal members 1 to be friction stirwelded, etc., and in many cases, the movement speed is set in the rangeof 30 to 300 (mm/min). When moving the primary welding rotating tool B,the axis line of the shoulder B1 may be inclined slightly to the backside of the traveling direction with respect to the vertical line,however, when the axis line is not inclined but set vertically, thetraveling direction of the primary welding rotating tool B can be easilychanged, and complicated movements thereof are possible.

When there is a possibility that the heat input into the metal members 1becomes excessive, preferably, cooling is performed by supplying waterto the surrounding of the primary welding rotating tool B from thesurface 12 side. If cooling water enters between the metal members 1, 1,it may form an oxide film on the joined surface (lateral side 11),however, in the present embodiment, the gap between the metal members 1,1 is closed by executing the preliminary joining process, so thatcooling water hardly enters between the metal members 1, 1, andtherefore, there is no possibility that the quality of the joinedportion is deteriorated.

At the butting portion J1 between the metal members 1, 1, a frictionstir welding route is set on the seam between the metal members 1, 1(the movement locus in the preliminary joining process), and byrelatively moving the primary welding rotating tool B along this route,friction stir welding is performed continuously from one end to theother end of the butting portion J1. After the primary welding rotatingtool B is relatively moved to the other end of the butting portion J1,it is made to pass across the butting portion J2 while performingfriction stir welding, and then relatively moved toward the end positionE_(M1).

In the present embodiment, the start position S_(M1) of friction stirwelding is set on the extension of the seam (boundary) between the metalmembers 1, 1 appearing on the surface 12 side of the metal members 1,therefore, the friction stir welding route in the first primary weldingprocess can be set straight. By forming the friction stir welding routestraight, the movement distance of the primary welding rotating tool Bcan be minimized, so that the first primary welding process can beefficiently performed, and further, a wear volume of the primary weldingrotating tool B can be reduced.

When the primary welding rotating tool B reaches the end positionE_(M1), as shown in FIG. 5( c), the primary welding rotating tool B ismoved up while being rotated to withdraw the stirring pin B2 from theend position E_(M1) (see FIG. 5( b)). When the stirring pin B2 iswithdrawn upward from the end position E_(M1), a pull-out hole Q1 havingsubstantially the same shape as the stirring pin B2 is inevitablyformed, however, this is left as it is in the present embodiment.

Preferably, the rotating speed of the primary welding rotating tool Bwhen withdrawing the stirring pin B2 of the primary welding rotatingtool B from the end position E_(M1) (rotating speed for withdrawal) isset to be higher than the rotating speed for movement. Accordingly, incomparison with the case where the rotating speed for withdrawal is setequal to the rotating speed for movement, the withdrawal resistance ofthe stirring pin B2 becomes smaller, so that the operation forwithdrawing the stirring pin B2 from the end position E_(M1) can bequickly performed.

If the rotating speed for withdrawal of the primary welding rotatingtool B is higher than 3.0 times the rotating speed for movement, theheat input into the metal increases and the temperature of the metalexcessively rises, and if the rotating speed for withdrawal is lowerthan 1.5 times the rotating speed for movement, the operation timeshortening effect becomes smaller, so that the rotating speed forwithdrawal of the primary welding rotating tool B is preferably set to1.5 to 3.0 times as high as the rotating speed for movement.

The withdrawal speed (moving-up speed) of the primary welding rotatingtool B is set according to the dimensions and shape of the stirring pinB2 and the material and thickness of the member on which the startposition S_(M1) is provided, etc., and in many cases, the withdrawalspeed is set in the range of 5 to 60 (mm/min).

(4) First Repairing Process

The first repairing process is for friction stir welding a surfaceplasticized region W1 formed on the metal members 1 by the first primarywelding process, and is performed for repairing joining defects whichmay be included in the surface plasticized region W1.

In the first repairing process of the present embodiment, as shown inFIG. 6( a) and FIG. 6( b), in the surface plasticized region W1, atleast a first repairing region R1, a second repairing region R2, and athird repairing region R3 are friction stir welded.

The friction stir welding on the first repairing region R1 is performedfor dividing a tunnel-like defect which may be formed along thetraveling direction of the primary welding rotating tool B. When theprimary welding rotating tool B is rotated clockwise, a tunnel-likedefect may occur on the left in the traveling direction, and when theprimary welding rotating tool B is rotated counterclockwise, atunnel-like defect may occur on the right in the traveling direction,and therefore, in the present embodiment in which the primary weldingrotating tool B is rotated clockwise, the first repairing region R1 ispreferably set so as to include at least an upper portion of the surfaceplasticized region W1 positioned on the left in the traveling directionin a plan view.

The friction stir welding on the second repairing region R2 is performedfor dividing an oxide film (oxide film formed on the contact surface 21between the lateral sides 14 of the metal members 1 and the first tabmember 2) involved in the surface plasticized region W1 when the primarywelding rotating tool B passes across the butting portion J2. In thecase where the end position E_(M1) of the friction stir welding of theprimary welding process is provided on the first tab member 2 as in thepresent embodiment, when the primary welding rotating tool B is rotatedclockwise, the oxide film is likely to be involved in the upper portionof the surface plasticized region W1 on the right in the travelingdirection, and when the primary welding rotating tool B is rotatedcounterclockwise, the oxide film is likely to be involved in the upperportion of the surface plasticized region W1 on the left in thetraveling direction, so that in the present embodiment in which theprimary welding rotating tool B is rotated clockwise, the secondrepairing region R2 is preferably set so as to include at least theupper portion of the surface plasticized region W1 positioned on theright in the traveling direction in a plan view in the surfaceplasticized region W1 adjacent to the first tab member 2. The distanced₄ from the seam between the metal members 1 and the first tab member 2to the edge on the metal member 1 side of the second repairing region R2is preferably set to be larger than the maximum outer diameter Y₂ of thestirring pin B2 of the primary welding rotating tool B.

The friction stir welding on the third repairing region R3 is performedfor dividing an oxide film (oxide film formed on the contact surface 31between the lateral sides 14 of the metal members 1 and the second tabmember 3) involved in the surface plasticized region W1 when the primarywelding rotating tool B passes across the butting portion J3. In thecase where the start position S_(M1) of the friction stir welding of theprimary welding process is provided on the second tab member 3 as in thepresent embodiment, when the primary welding rotating tool B is rotatedclockwise, the oxide film is likely to be involved in the upper portionof the surface plasticized region W1 on the left in the travelingdirection, and when the primary welding rotating tool B is rotatedcounterclockwise, the oxide film is likely to be involved in the upperportion of the surface plasticized region W1 on the right in thetraveling direction, so that in the present embodiment in which theprimary welding rotating tool B is rotated clockwise, the thirdrepairing region R3 is preferably set so as to include at least theupper portion of the surface plasticized region W1 positioned on theleft in the traveling direction in a plan view in the surfaceplasticized region W1 adjacent to the second tab member 3. The distanced₅ from the seam between the metal members 1 and the second tab member 3to the edge on the metal member 1 side of the third repairing region R3is preferably set to be larger than the maximum outer diameter Y₂ of thestirring pin B2 of the primary welding rotating tool B.

In the first repairing process of the present embodiment, friction stirwelding is performed by using a repairing rotating tool C smaller thanthe primary welding rotating tool B. Accordingly, the plasticized regioncan be prevented from excessively spreading.

Similar to the preliminary joining rotating tool A, the repairingrotating tool C is made of a metal material such as tool steel harderthan the metal members 1, and as shown in FIG. 6( b), includes ashoulder C1 in a columnar shape and a stirring pin (probe) C2 providedso as to project from the lower end surface of the shoulder C1.

The stirring pin C2 is hung down from the lower end surface of theshoulder C1, and in the present embodiment, formed into a circulartruncated cone shape. On the peripheral surface of the stirring pin C2,a stir wing carved spirally is formed. In many cases, joining defectsdue to the primary welding rotating tool B shown in FIG. 2( b) areformed in the range to ⅓ from the upper end of the stirring pin B2, sothat the length of the stirring pin C2 of the repairing rotating tool Cis preferably set to not less than ⅓ of the length L₁ (see FIG. 2( b))of the stirring pin B2 of the primary welding rotating tool B, and ifthe length is more than ½, the plasticized region may excessivelyspread, so that the length is preferably set to not more than ½. Themaximum outer diameter (upper end diameter) and the minimum outerdiameter (lower end diameter) of the stirring pin C2 are notspecifically limited, however, in the present embodiment, they aresmaller than the maximum outer diameter (upper end diameter) Y₂ and theminimum outer diameter (lower end diameter) Y₃ of the stirring pin B2 ofthe primary welding rotating tool B, respectively.

In the first repairing process, each time the friction stir welding onone repairing region is finished, the repairing rotating tool C may bewithdrawn, or a different repairing rotating tool C may be used for eachrepairing region, however, in the present embodiment, as shown in FIG.7, one repairing rotating tool C is moved so as to form a one-strokemovement locus (bead) to continuously apply friction stir welding to thefirst repairing region R1, the second repairing region R2, and the thirdrepairing region R3. In other words, in the first repairing process ofthe present embodiment, the stirring pin C2 (see FIG. 6( b)) of therepairing rotating tool C inserted into the start position S_(R) offriction stir welding is moved to the end position E_(R) without beingwithdrawn midway. In the present embodiment, a case where the startposition S_(R) of the friction stir welding is provided on the first tabmember 2, the end position E_(R) is provided on the second tab member 3,and the second repairing region R2, the first repairing region R1, andthe third repairing region R3 are friction stir welded in this order, isillustrated, however, this is not intended to limit the positions of thestart position S_(R) and the end position E_(R) and the friction stirwelding order.

Friction stir welding steps in the first repairing process will bedescribed in detail with reference to FIG. 7. First, the stirring pin C2(see FIG. 6( b)) of the repairing rotating tool C is inserted(press-fitted) to the start position S_(R) provided on an appropriatelocation on the first tab member 2 and friction stir welding is started,and the second repairing region R2 is friction stir welded.

The rotating speed and the insertion speed (moving-down speed) wheninserting the repairing rotating tool C into a start position S_(R) areset according to the dimensions and shape of the stirring pin C2 and thematerials and thicknesses of the metal members 1 to be friction stirwelded, etc., and in many cases, the rotating speed is set in the rangeof 300 to 2000 (rpm), and the insertion speed is set in the range of 30to (mm/min).

The movement speed (feed rate) of the repairing rotating tool C is setaccording to the dimensions and shape of the stirring pin C2 and thematerials and thicknesses of the metal members 1 to be friction stirwelded, etc., and in many cases, the movement speed is set in the rangeof 100 to 1000 (mm/min). The rotating speed for movement of therepairing rotating tool C is set to be equal to or lower then therotating speed for insertion.

When the second repairing region R2 is friction stir welded, even if theoxide film on the contact surface 21 between the lateral sides 14 of themetal members 1 and the first tab member 2 is involved in the surfaceplasticized region W1 (see FIG. 6( b)), this oxide film can be divided,therefore, joining defects hardly occur in the surface plasticizedregion W1 adjacent to the first tab member 2. When the second repairingregion R2 is larger than the region which can be friction stir welded bythe repairing rotating tool C, the repairing rotating tool C is turnedaround several times while shifting the friction stir welding route.

When the friction stir welding on the second repairing region R2 isfinished, the repairing rotating tool C is directly moved to the firstrepairing region R1 without being withdrawn to perform friction stirwelding continuously along the friction stir welding route in the firstprimary welding process described above. Accordingly, even when atunnel-like defect is formed continuously along the friction stirwelding route in the primary welding process, this tunnel-like defectcan be reliably divided, so that joining defects hardly occur.

When the friction stir welding on the first repairing region R1 isfinished, the repairing rotating tool C is directly moved to the thirdrepairing region R without being withdrawn, and the third repairingregion R3 is friction stir welded. Accordingly, even when an oxide filmon the contact surface 31 between the lateral sides 14 of the metalmembers 1 and the second tab member 3 is involved in the surfaceplasticized region W1 (see FIG. 6( b)), this oxide film can be divided,so that joining defects hardly occur in the surface plasticized regionW1 adjacent to the second tab member 3. When the third repairing regionR3 is larger than the region which can be friction stir welded by therepairing rotating tool C, the repairing rotating tool C is turnedaround several times while shifting the friction stir welding route.

When the friction stir welding on the third repairing region R3 isfinished, the repairing rotating tool C is moved to the end positionE_(R), and the stirring pin C2 (see FIG. 6( b)) is withdrawn from theend position E_(R) by moving-up the repairing rotating tool C whilerotating it.

The rotating speed and the withdrawal speed (moving-up speed) when therepairing rotating tool C is withdrawn from the end position E_(R) areset according to the dimensions and shape of the stirring pin C2 and thematerials and thicknesses of the metal materials 1 to be friction stirwelded, and in many cases, the rotating speed is set in the range of 300to 2000 (rpm) and the withdrawal speed is set in the range of 30 to 60(mm/min). The rotating speed for withdrawal of the repairing rotatingtool C is set to be equal to or higher than the rotating speed formovement.

(5) First Traverse Repairing Process

The first traverse repairing process is also for friction stir weldingthe surface plasticized region W1 formed on the metal members 1 by thefirst primary welding process, and is executed for dividing atunnel-like defect which is likely to be included in the surfaceplasticized region W1.

In the first traverse repairing process of the present embodiment, asshown in FIG. 8, by moving the traversing rotating tool D so as totraverse the surface plasticized region W1 a plurality of times, thesurface plasticized region W1 is friction stir welded. That is, in thefirst traverse repairing process, a friction stir welding route is setso as to traverse the surface plasticized region W1 a plurality oftimes. Accordingly, even when a tunnel-like defect is formed along thesurface plasticized region W1, the tunnel-like defect can be dividedwith satisfactory reliability.

The friction stir welding route in the first traverse repairing processis set so that a plurality of plasticized regions (hereinafter, referredto as “re-plasticized regions”) W3, W3 . . . formed in the surfaceplasticized region W1 are spaced from each other on the friction stirwelding route (that is, the center line of the surface plasticizedregion W1) in the first primary welding process.

In the friction stir welding route in the first traverse repairingprocess, a plurality of traverse routes F1 which traverse the surfaceplasticized region W1 and transfer routes F2 which connect end portionson the same side of traverse routes F1, F1 adjacent to each other, areprovided. In other words, the friction stir welding route in the firsttraverse repairing process includes at least a first traverse route F1set so as to start from the lateral of the surface plasticized region W1toward the opposite side across the surface plasticized region W1, atransfer route F2 set so as to start from an end point e10 of thetraverse route F1 and disposed along the friction stir welding route(seam between the metal members 1, 1) in the first primary weldingprocess, and a second traverse route F1 set so as to start from an endpoint s10 of the transfer route F2 toward the opposite side across thesurface plasticized region W1.

The traverse route F1 is a friction stir welding route set so as totraverse the surface plasticized region W1, and in the presentembodiment, crosses the friction stir welding route of the first primarywelding process at right angles. The start point s10 and the end pointe10 of the traverse route F1 are positioned lateral to the surfaceplasticized region W1, and are opposed to each other across the surfaceplasticized region W1.

The positions of the start point s10 and the endpoint e10 of thetraverse route F1 are preferably set so that the entirety of thetraversing rotating tool D comes out from the surface plasticized regionW1, however, if they are excessively apart from the surface plasticizedregion W1, the movement distance of the traversing rotating tool Dincreases, so that in the present embodiment, the distance from thestart point s10 to the lateral edge of the surface plasticized region W1and the distance from the lateral edge of the surface plasticized regionW1 to the end point e10 become equal to the half of the outer diameterX₄ (see FIG. 9) of the shoulder D2 of the traversing rotating tool D. Inother words, the length of the traverse route F1 (distance from thestart point s10 to the end point e10) becomes equal to a value obtainedby adding the outer diameter X₄ of the shoulder D2 to the width d₆ ofthe surface plasticized region W1. Incidentally, the width d₉ of theplasticized region formed by the traversing rotating tool D becomessubstantially equal to the outer diameter X₄ of the shoulder D2,therefore, the length of the traverse route F1 is substantially equal tothe value obtained by adding the width d₉ of the plasticized regionformed by the traversing rotating tool D to the width d₆ of the surfaceplasticized region W1.

The spacing distance d₇ between the traverse routes F1, F1 adjacent toeach other is set to a size so that the re-plasticized regions W3, W3 .. . are spaced from each other on the friction stir welding route of thefirst primary welding process (center line of the surface plasticizedregion W1). The spacing distance d₈ between the re-plasticized regionsW3, W3 adjacent to each other is secured preferably not less than thewidth d₉ of the re-plasticized regions W3, more preferably, not lessthan two times the width d₉.

The transfer route F2 is a friction stir welding route from the endpoint e10 of one traverse route F1 to the start point s10 of anothertraverse route F1 positioned closer to the friction stir welding endposition E_(C) side than the one traverse route F1, and in the presentembodiment, provided on the right or left of the surface plasticizedregion W1 and parallel to the friction stir welding route of the firstprimary welding process.

The transfer route F2 is preferably set at a position at which aplasticized region W4 formed by moving the traversing rotating tool Dalong the transfer route F2 comes into contact with the lateral edge ofthe surface plasticized region W1. In the present embodiment, asdescribed above, the distance between the end point e10 of the traverseroute F1 which is the start point of the transfer route F2 and thelateral edge of the surface plasticized region W1 and the distancebetween the start point s10 of the traverse route F1 which is the endpoint of the traverse route F2 and the lateral edge of the surfaceplasticized region W1 are equal to the half of the width d₉ of theplasticized region formed by the traversing rotating tool D, so that theplasticized region W4 inevitably comes into contact with the lateraledge of the surface plasticized region W1.

Friction stir welding steps in the first traverse repairing process willbe described in detail.

In the present embodiment, the repairing rotating tool C (see FIG. 6( a)and FIG. 6( b)) is used as the traversing rotating tool D, and detaileddescription thereof is omitted. The rotating speed, the insertion speed,the movement speed, and the withdrawal speed of the traversing rotatingtool D are the same as those of the repairing rotating tool C, so thatdetailed description of these is omitted.

In the present embodiment, by moving one traversing rotating tool Dzigzag so as to form a one-stroke movement locus (bead), friction stirwelding is performed continuously from the start position S_(C) to theend position E_(C) of friction stir welding. In other words, thestirring pin D2 (see FIG. 9) of the traversing rotating tool D insertedinto the start position S_(C) of friction stir welding is moved to theend position E_(C) without being withdrawn midway. It is also allowedthat the traversing rotating tool D is withdrawn each time that ittraverses the surface plasticized region W1. In the present embodiment,the case where the start position S_(C) of friction stir welding isprovided on the metal member 1 and the end position E_(C) is provided onthe second tab member 3 is illustrated, however, this is not intended tolimit the positions of the start position S_(C) and the end positionE_(C).

Friction stir welding steps in the first traverse repairing process willbe described in detail.

In the first traverse repairing process, first, the stirring pin D2 (seeFIG. 9) of the traversing rotating tool D is inserted (press-fitted) tothe start position S_(C) provided at an appropriate position on themetal member 1 and friction stir welding is started, and friction stirwelding is performed continuously along the first traverse route F1.

When the traversing rotating tool D is moved along the traverse routeF1, the metal on the upper portion of the surface plasticized region W1is friction stir welded again (see FIG. 9), so that a tunnel-like defectwhich cannot be completely divided by the first repairing process and atunnel-like defect formed out of the friction stir welding route of thefirst repairing process can be divided with satisfactory reliability.

When the traversing rotating tool D reaches the end point e10 of thefirst traverse route F1, the traveling direction of the traversingrotating tool D is changed to a direction along the friction stirwelding route of the primary welding process, and the traversingrotating tool D is moved along the transfer route F2. In other words,the traversing rotating tool D is not withdrawn at the endpoint e10 butmoved continuously along the transfer route F2 to continuously frictionstir weld the metal lateral to the surface plasticized region W1. In thepresent embodiment, the transfer route F2 is parallel to the frictionstir welding route of the first primary welding process, so that thedistance of the transfer route F2 is shorter than in the case where thetransfer route F2 is inclined.

When the traversing rotating tool D reaches the start point s10 of thesecond traverse route F1, the traveling direction of the traversingrotating tool D is changed to a direction crossing the surfaceplasticized region W1, and the traversing rotating tool D is movedcontinuously along the second traverse route F1 without being withdrawnto continuously friction stir weld the surface plasticized region W1.

After repeating the above-described steps, when the traversing rotatingtool D reaches the end point e10 of the last traverse route F1, thetraversing rotating tool D is moved to the end position E_(C), and thestirring pin D2 (see FIG. 9) is withdrawn from the end position E_(C) bymoving-up the traversing rotating tool D while rotating it.

Thus, by spacing the plurality of re-plasticized regions W3, W3 . . .from each other on the friction stir welding route of the first primarywelding process, in comparison with the case where the re-plasticizedregions are formed in the entire surface plasticized region W1, thenumber of traverses and the number of direction changes of thetraversing rotating tool D become smaller, and as a result, the totalextension of the friction stir welding route of the first traverserepairing process becomes shorter, therefore, wasteful movements of thetraversing rotating tool D can be prevented, and eventually, atunnel-like defect can be efficiently divided. Particularly, in thepresent embodiment, the spacing distance d₈ between the re-plasticizedregions W3, W3 adjacent to each other is set to not less than the widthd₉ of the re-plasticized regions W3, therefore, wasteful movements ofthe traversing rotating tool D can be further prevented.

A rotating tool different from the repairing rotating tool C may be usedas the traversing rotating tool D, however, even in this case, it ispreferably smaller than the primary welding rotating tool B shown inFIG. 2( a) and FIG. 2( b). A tunnel-like defect caused by the primarywelding rotating tool B is formed in the range to ⅓ from the upper endof the stirring pin in many cases, so that the length of the stirringpin of the traversing rotating tool D is preferably set to not less than⅓ of the length L₁ of the stirring pin B2 (see FIG. 2( a) and FIG. 2(b)) of the primary welding rotating tool B, and if it is larger than ½,the plasticized region may excessively spread, so that the length ispreferably set to not more than ½.

When the first traverse repairing process is finished, burrs formedduring friction stir welding in the first preparatory process, the firstprimary welding process, the first repairing process, and the firsttraverse repairing process are removed, and further, as shown in FIG.10( a), the metal members 1, 1 are turned over so that the back surfaces13 thereof turn up.

(6) Second Preparatory Process

The second preparatory process is performed previous to the secondprimary welding process, and in the present embodiment, includes a pilotbore forming process in which a pilot bore P2 is formed at the startposition S_(M2) of friction stir welding of the second primary weldingprocess. The above-described first tab member joining process, thepreliminary joining process, and the second tab member joining processmay be included in the second preparatory process.

(7) Second Primary Welding Process

The second primary welding process is for primarily welding the buttingportion J1 between the metal members 1, 1. In the second primary weldingprocess of the present embodiment, as shown in FIG. 10( a) and FIG. 10(b), the butting portion J1 is friction stir welded from the back surface13 side of the metal members 1 by using the primary welding rotatingtool B used in the first primary welding process.

In the second primary welding process, the stirring pin B2 of theprimary welding rotating tool B is inserted (press-fitted) into thepilot bore P2 (start position S_(M2)) disposed in the second tab member3, and the inserted stirring pin B2 is moved to the end position E_(M2)provided on the first tab member 2 without being withdrawn midway. Inother words, in the second primary welding process, friction stirwelding is started from the pilot bore P2 and continued to the endposition E_(M2).

The second primary welding process will be described in detail withreference to FIG. 10( a) to FIG. 10( c).

First, as shown in FIG. 10( a), the primary welding rotating tool B ispositioned just above the pilot bore P2, and then, the primary weldingrotating tool B is moved down while being rotated clockwise to insertthe tip end of the stirring pin B2 into the pilot bore P2. The rotatingspeed for insertion of the primary welding rotating tool B is preferablyset to be higher than the rotating speed for movement of the primarywelding rotating tool B as in the case of the first primary weldingprocess described above.

When the entire stirring pin B2 gets into the second tab member 3 andthe entire lower end surface B11 of the shoulder B1 comes into contactwith the surface of the second tab member 3, as shown in FIG. 10( b),the primary welding rotating tool B is relatively moved toward one endof the butting portion J1 between the metal members 1, 1 while frictionstir welding. When the primary welding rotating tool B is moved, themetal around the stirring pin B2 is successively plasticized andfluidized, and at a position apart from the stirring pin B2, theplasticized and fluidized metal hardens again and forms a plasticizedregion W2 (hereinafter, referred to as “back plasticized region W2”). Inthe present embodiment, the same primary welding rotating tool B is usedin the first primary welding process and the second primary weldingprocess, so that the cross section area of the back plasticized regionW2 becomes equal to that of the surface plasticized region W1. As in thecase of the first primary welding process, when the primary weldingrotating tool B is moved, the axis line of the shoulder B1 may beinclined slightly to the back side of the traveling direction withrespect to the vertical line, however, when the axis line is notinclined but set vertically, the traveling direction of the primarywelding rotating tool B can be easily changed, and complicated movementsthereof are possible. When the heat input into the metal members 1 maybecome excessive, preferably, the surrounding of the primary weldingrotating tool B is cooled by supplying water from the back surface 13side.

When the primary welding rotating tool B reaches one end of the buttingportion J1 between the metal members 1, 1, it is relatively moved alongthe seam between the metal members 1, 1 to continuously perform frictionstir welding to the other end of the butting portion J1, and further,the primary welding rotating tool B is relatively moved to the endposition E_(M2) while friction stir welding.

When the butting portion J1 is friction stir welded, friction stirwelding is performed while inserting the stirring pin B2 of the primarywelding rotating tool B into the surface plasticized region W1 formedthrough the first primary welding process. Accordingly, the deep portionof the surface plasticized region W1 formed through the first primarywelding process is friction stir welded again by the stirring pin B2, sothat even when joining defects are formed continuously in the deepportion of the surface plasticized region W1, the joining defects can bedivided so as to become discontinuous, and eventually, airtightness andwater-tightness of the joined portion can be improved. In the presentembodiment, the length L₁ of the stirring pin B2 of the primary weldingrotating tool B is set so as to satisfy the relation of 1.01≦2L₁/t≦1.10(see FIG. 2( b)), so that simply by moving the primary welding rotatingtool B along the seam between the metal members 1, 1, the stirring pinB2 gets into the surface plasticized region W1 reliably.

When the primary welding rotating tool B reaches the end positionE_(M2), the stirring pin B2 is withdrawn from the end position E_(M2) bymoving up the primary welding rotating tool B while rotating it (seeFIG. 10( c)). The rotating speed for withdrawal of the primary weldingrotating tool B is preferably set to be higher than the rotating speedfor movement as in the case of the first primary welding process.

When the pull-out hole Q1 left in the first primary welding process andthe movement route of the primary welding rotating tool B in the secondprimary welding process overlap each other, the plasticized andfluidized metal may flow into the pull-out hole Q1 and cause joiningdefects, so that preferably, the end position E_(M2) of friction stirwelding (pull-out hole Q2) of the second primary welding process isprovided apart from the pull-out hole Q1, and the friction stir weldingroute of the second primary welding process is set so as to avoid thepull-out hole Q1, and the stirring pin B2 of the primary weldingrotating tool B is moved along this route.

Even when the stirring pin B2 of the primary welding rotating tool B tobe used in the second primary welding process does not pass through thepull-out hole Q1 of the first primary welding process, if the spacingdistance between these is small, the plasticized and fluidized metal maybe pushed out to the pull-out hole Q1 and cause joining defects, so thatmore preferably, the shortest distance d₁ in a plan view between the endposition E_(M1) of friction stir welding of the first primary weldingprocess and the movement locus of the primary welding rotating tool B ofthe second primary welding process (end position E_(M2) in the presentembodiment) is preferably set to not less than the outer diameter of theshoulder B1 of the primary welding rotating tool B.

As described in the present embodiment, by performing the second primarywelding process by using the primary welding rotating tool B used in thefirst primary welding process, the operation efficiency is improved andthe cost can be reduced, and further, the cross section area of thesurface plasticized region W1 and the cross section area of the backplasticized region W2 become equal to each other, so that the quality ofthe joined portion becomes uniform, however, the primary weldingrotating tool is allowed to be different between the first primarywelding process and the second primary welding process.

When the primary welding rotating tool to be used is different betweenthe first primary welding process and the second primary weldingprocess, for example, as shown in FIG. 11( a) and FIG. 11( b),preferably, the sum of the length L₁ of the stirring pin B2 of theprimary welding rotating tool B to be used in the first primary weldingprocess and the length L₂ of the stirring pin B2′ of the primary weldingrotating tool B′ to be used in the second primary welding process is setto not less than the thickness t of the metal members 1 at the buttingportion J1. As a matter of course, each of the lengths L₁, L₂ of thestirring pins B2, B2′ is less than the thickness t. Accordingly, thedeep portion of the surface plasticized region W1 formed through thefirst primary welding process is friction stir welded again by thestirring pin B2′ of the primary welding rotating tool B′ used in thesecond primary welding process, so that even if joining defects areformed continuously at the deep portion of the surface plasticizedregion W1, the joining defects can be divided to be discontinuous, andeventually, airtightness and water-tightness at the joined portion canbe improved.

More preferably, as shown in FIG. 11( a) and FIG. 11( b), each of thelengths L₁, L₂ of the stirring pins B2, B2′ of the primary weldingrotating tool B and B′ is preferably set to not less than ½ of thethickness t of the metal members 1 at the butting portion J1 and notmore than ¾ of the thickness t. By setting each of the lengths L₁, L₂ ofthe stirring pins B2, B2′ to not less than ½ of the thickness t, thesurface plasticized region W1 and the back plasticized region W2 overlapat the central portion in the thickness direction of the metal members1, and the difference in cross section area between the surfaceplasticized region W1 and the back plasticized region W2 becomessmaller, so that the quality at the joined portion becomes uniform, andby setting each of the lengths L₁, L₂ of the stirring pins B2, B2′ tonot more than ¾ of the thickness t, a backing material becomesunnecessary when performing friction stir welding, so that the operationefficiency can be improved.

Still more preferably, the length L₁, L₂ of the stirring pins B2, B2′are set so as to satisfy the relation of 1.01≦(L₁+L₂)/t≦1.10. By setting(L₁+L₂)/t to not less than 1.01, even if the metal members 1 havedimensional tolerances, etc., the stirring pin B2′ can be reliably madeto get into the surface plasticized region W1 in the second primarywelding process. When (L₁+L₂)/t is set to be larger than 1.10, therotating tools become excessively large and the load on the frictionstir welding machine increases, however, by setting (L₁+L₂)/t to notmore than 1.10, the load on the friction stir welding machine becomessmaller.

(8) Second Repairing Process

The second repairing process is for friction stir welding the backplasticized region W2 formed on the metal members 1 through the secondprimary welding process, and is performed for repairing joining defectswhich are likely to be included in the back plasticized region W2. Thesecond repairing process is the same as the first repairing processdescribed above except that the friction stir welding is performed fromthe back surface 13 side of the metal members 1, so that detaileddescription of the second repairing process is omitted.

(9) Second Traverse Repairing Process

The second traverse repairing process is for friction stir welding aback plasticized region W2 formed on the metal members 1 through thesecond primary welding process, and is executed for dividing atunnel-like defect which is likely to be included in the backplasticized region W2. The second traverse repairing process is the sameas the first traverse repairing process described above except that thefriction stir welding is performed from the back surface 13 side of themetal members 1, so that detailed description of the second traverserepairing process is omitted.

After the second traverse repairing process is finished, burrs formedduring friction stir welding in the second preparatory process, thesecond primary welding process, the second repairing process, and thesecond traverse repairing process are removed, and further, the firsttab member 2 and the second tab member 3 are cut off.

By performing the above-described processes (1) to (9), even when verythick metal members 1, 1 whose thicknesses are more than 40 (mm) arejoined, airtightness and water-tightness at the joined portion can beimproved.

MODIFIED EXAMPLE

The steps, etc., of the first embodiment described above may be changedas appropriate.

For example, in the first tab member joining process, the positions ofthe start point s2 and the end point e2 of the first tab member joiningprocess are set so that the metal members 1, 1 are positioned on theright in the traveling direction of the preliminary joining rotatingtool A which is rotated clockwise (see FIG. 4), however, withoutlimiting to this, it is also allowed that the position of the end pointe2 shown in FIG. 4 is set as a turning point and the position of thestart point s1 of the preliminary joining process shown in FIG. 4 is setas the end point of the first tab member joining process. In otherwords, as shown in FIG. 12, it is also allowed that the start point s2and the turning point m2 are provided on the seam between the metalmembers 1 and the first tab member 2, the start point s1 of thepreliminary joining process between the start point s2 and the turningpoint m2 is set as the end point e2, and after the preliminary joiningrotating tool is moved from the start point s2 to the turning point m2,the preliminary joining rotating tool is moved from the turning point m2to the end point e2 to friction stir weld the butting portion J2.

Accordingly, it is not necessary to set the friction stir welding routefrom the end point e2 of the first tab member joining process to thestart point s1 of the preliminary joining process on the first tabmember 2, so that the movement distance of the preliminary joiningrotating tool A can be minimized. In other words, the first tab memberjoining process can be efficiently performed, and the wear volume of thepreliminary joining rotating tool A can be reduced.

When the preliminary joining rotating tool A is rotated clockwise,preferably, the positions of the start point s2, the turning point m2,and the end point e2 of the first tab member joining process are set sothat the metal members 1, 1 are positioned on the right in the travelingdirection of the preliminary joining rotating tool A at least in thefriction stir welding route from the start point s2 to the turning pointm2. Incidentally, when the preliminary joining rotating tool A isrotated counterclockwise, preferably, the positions of the start point,the turning point, and the end point of the first tab member joiningprocess are set so that the metal members 1, 1 are positioned on theleft in the traveling direction of the preliminary joining rotating toolA at least in the friction stir welding route from the start point tothe turning point although these are not shown.

In the second tab member joining process, the positions of the startpoint s3 and the end point e3 of the second tab member joining processare also set so that the metal members 1, 1 are positioned on the rightin the traveling direction of the preliminary joining rotating tool Awhich is rotated clockwise (see FIG. 4), however, without limiting tothis, it is also allowed that the end point s1 of the preliminaryjoining process shown in FIG. 4 is set as the start point and theposition of the start point s3 of the second tab member joining processshown in FIG. 4 is set as the turning point. In other words, as shown inFIG. 12, it is allowed that the turning point m3 and the end point e3are provided on the seam between the metal members 1 and the second tabmember 3, the end point e1 of the preliminary joining process betweenthe turning point m3 and the end point e3 is set as the start point s3,and by moving the preliminary joining rotating tool A from the turningpoint m3 to the end point e3 after moving the preliminary joiningrotating tool A from the start point s3 to the turning point m3, thebutting portion J3 is friction stir welded.

Accordingly, it is not necessary to set the friction stir welding routefrom the end point e1 of the preliminary joining process to the startpoint s3 of the second tab member joining process on the second tabmember 3, so that the movement distance of the preliminary joiningrotating tool A can be minimized. In other words, the second tab memberjoining process can be efficiently performed, and the wear volume of thepreliminary joining rotating tool A can be reduced.

When the preliminary joining rotating tool A is rotated clockwise,preferably, the positions of the start point s3, the turning point m3,and the end point e3 of the second tab member joining process are set sothat the metal members 1, 1 are positioned on the right in the travelingdirection of the preliminary joining rotating tool A at least in thefriction stir welding route from the turning point m3 to the end pointe3. Incidentally, when the preliminary joining rotating tool A isrotated counterclockwise, preferably, the start point, the turningpoint, and the end point of the second tab member joining process areset so that the metal members 1, 1 are positioned on the left in thetraveling direction of the preliminary joining rotating tool A at leastin the friction stir welding route from the turning point to the endpoint although these are not shown.

In the repairing process described above, the first repairing region R1,the second repairing region R2, and the third repairing region R3 arefriction stir welded (see FIG. 7), however, it is also allowed that onlythe second repairing region R2 and the third repairing region R3 arefriction stir welded.

In this case, as shown in FIG. 13, on each of both end portions of thebutting portion J1, a repairing rotating tool C′ is moved so as totraverse the plasticized region (second repairing region R2 or thirdrepairing region R3) formed through the primary welding process. Inother words, at one end portion of the butting portion J1, the secondrepairing region R2 is friction stir welded by moving the repairingrotating tool C′ along the butting portion J2, and at the other endportion of the butting portion J1, the third repairing region R3 isfriction stir welded by moving the repairing rotating tool along thebutting portion J3. In detail, the start position S_(R) of friction stirwelding is provided on one metal member 1, and by moving the repairingrotating tool toward the other metal member 1, the lateral edge portionof the metal members 1 is friction stir welded. Even in this case, oxidefilms involved by the primary welding rotating tool B (see FIG. 2( b))when it passes across the butting portions J2 and J3 are divided, sothat a joined body with very few joining defects can be obtained.

In the case of friction stir welding along the butting portion J2, whenthe repairing rotating tool C′ is rotated clockwise, the friction stirwelding route is set so that the first tab member 2 is positioned on theleft in the traveling direction, and when the repairing rotating tool isrotated counterclockwise as shown in the figure, the friction stirwelding route is set so that the first tab member 2 is positioned on theright in the traveling direction. Similarly, in the case of frictionstir welding along the butting portion J3, when the repairing rotatingtool C′ is rotated clockwise as shown in the figure, the friction stirwelding route is set so that the second tab member 3 is positioned onthe left in the traveling direction, and when the repairing rotatingtool is rotated counterclockwise, the friction stir welding route is setso that the second tab member 3 is positioned on the right in thetraveling direction. In either case, the end position E_(R) of frictionstir welding is provided on the first tab member 2 or the second tabmember 3 so as to prevent pull-out holes of the stirring pin of therepairing rotating tool C′ from remaining in the metal members 1, 1.

In the above-described traverse repairing process, the case where thetraverse routes F1 as a part of the friction stir welding route are madecross the friction stir welding route of the primary welding process atright angles is illustrated (see FIG. 8), however, diagonal crossinglike the traverse routes F1′ shown in FIG. 14 is also allowed.

The friction stir welding route of the traverse repairing process shownin FIG. 14 is also set so that the plurality of re-plasticized regionsW3, W3 . . . are spaced from each other on the friction stir weldingroute (center line of the plasticized region W1) of the primary weldingprocess, however, the transfer routes F2 shown in FIG. 8 are omitted anda plurality of traverse routes F1′, F1′ . . . are formed continuously.Preferably, the spacing distance d₈ between re-plasticized regions W3,W3 adjacent to each other on the center line of the plasticized regionW1 is secured not less than the width d₉ of the re-plasticized regionsW3, and more preferably, not less than 2 times the width d₉.

The traverse routes F1′ are also set so as to traverse the plasticizedregion W1 formed through the primary welding process. The start point(which is the end point of the previous traverse route F1′) s11 and theend point (which is the start point of the next traverse route F1′) ellof the traverse route F1′ are positioned lateral to the plasticizedregion W1, and are opposed to each other across the plasticized regionW1.

Preferably, the start point s11 and the end point e11 of the traverseroute F1′ are set at the positions so that the entirety of thetraversing rotating tool D comes out from the plasticized region W1,however, if they are set at positions excessively apart from theplasticized region W1, the movement distance of the traversing rotatingtool D increases, therefore, in the present embodiment, they are set sothat the distance from the start point s11 to the lateral edge of theplasticized region W1 and the distance from the lateral edge of theplasticized region W1 to the end point e11 are equal to the half of theouter diameter of the shoulder of the traversing rotating tool D.

In the first traverse repairing process of the modified example, first,the stirring pin of the traversing rotating tool D is inserted(press-fitted) to the start position S_(C) provided at an appropriatelocation on the metal members 1 and friction stir welding is started,and friction stir welding is performed continuously along the firsttraverse route F1′.

When the traversing rotating tool D reaches the end point ell of thefirst traverse route F1′, the traveling direction of the traversingrotating tool D is changed and the traversing rotating tool is movedalong the second traverse route F1′. In other words, the traversingrotating tool D is not withdrawn at the end point e11 but movedcontinuously along the second traverse route F1′ to continuouslyfriction stir weld the plasticized region W1.

After repeating these steps, when the traversing rotating tool D reachesthe end point e11 of the last traverse route F1′, the traversingrotating tool D is moved to the end position E_(C), and the stirring pinis withdrawn from the end position E_(C) by moving up the traversingrotating tool D while rotating it.

As in the case of the traverse repairing process shown in FIG. 14, bycontinuing the plurality of traverse routes F1′, F1′ . . . crossingdiagonally the plasticized region W1 zigzag, the number of directionchanges of the traversing rotating tool D can be made smaller than inthe case where transfer routes are provided (see FIG. 8), therefore,wasteful movements of the traversing rotating tool D can be furtherprevented, and a tunnel-like defect can be more effectively divided.

Second Embodiment

In the above-described first embodiment, the case where the metalmembers 1, 1 are joined straight is illustrated, however, theabove-described method can be applied even when the metal members 1, 1are joined in an L shape or a T shape. Hereinafter, the case where themetal members 1, 1 are joined in an L shape will be illustrated.

Similar to the joining method of the first embodiment, the joiningmethod of the second embodiment also includes: (1) a preparationprocess, (2) a first preparatory process, (3) a first primary weldingprocess, (4) a first repairing process, (5) a first traverse repairingprocess, (6) a second preparatory process, (7) a second primary weldingprocess, (8) a second repairing process, and (9) a second traverserepairing process. The first preparatory process, the first primarywelding process, the first repairing process, and the first traverserepairing process are executed from the surface side of the metalmembers 1, and the second preparatory process, the second primarywelding process, the second repairing process, and the second traverserepairing process are executed from the back surface side of the metalmembers 1.

(1) Preparation Process

The preparation process will be described with reference to FIG. 15( a)and FIG. 15( b). The preparation process of the present embodimentincludes a butting process in which metal members 1, 1 to be joined arebutted against each other, a tab member disposing process in which afirst tab member 2 and a second tab member 3 are disposed on both sidesof the butting portion J1 of the metal members 1, 1, and a weldingprocess in which the first tab member 2 and the second tab member 3 arepreliminarily joined to the metal members 1, 1 by welding.

In the butting process, metal members 1, 1 to be joined are disposed inan L shape, and with the lateral side of one metal member 1, the lateralside of the other metal member 1 is brought into close contact.

In the tab member disposing process, the first tab member 2 is disposedon one end side (outer side) of the butting portion J1 between the metalmembers 1, 1 and the contact surface 21 (see FIG. 15( b)) of the firsttab member 2 is brought into contact with the outer lateral sides of themetal members 1, 1, and the second tab member 3 is disposed on the otherend side of the butting portion J1 and the contact surfaces 31, 31 (seeFIG. 15( b)) of the second tab member 3 are brought into contact withthe inner lateral sides of the metal members 1, 1. When the metalmembers 1, 1 are fitted in an L shape, either (the second tab member 3in the present embodiment) of the first tab member 2 and the second tabmember 3 is disposed at the inner corner portion formed by the metalmembers 1, 1 (an angulated portion formed by the inner lateral sides ofthe metal members 1, 1).

In the welding process, the inner corner portions 2 a, 2 a formed by themetal members 1 and the first tab member 2 are welded to join the metalmembers 1 and the first tab member 2, and the inner corner portions 3 a,3 a formed by the metal members 1 and the second tab member 3 are weldedto join the metal members 1 and the second tab member 3.

When the preparation process is finished, the metal members 1, 1, thefirst tab member 2, and the second tab member 3 are placed on a platformof a friction stir welding machine not shown, and locked so as not tomove by using a jig such as a clamp not shown.

(2) First Preparatory Process

The first preparatory process includes a first tab member joiningprocess in which the butting portion J2 between the metal members 1, 1and the first tab member 2 is joined, a preliminary joining process inwhich the butting portion J1 between the metal members 1, 1 ispreliminary joined, a second tab member joining process in which thebutting portion J3 between the metal members 1, 1 and the second tabmember 3 is joined, and a pilot bore forming process in which a pilotbore is formed at a start position of friction stir welding of the firstprimary welding process.

In the first preparatory joining process of the present embodiment, asshown in FIG. 16( a) and FIG. 16( b), the butting portions J1, J2, andJ3 are also successively friction stir welded by moving one preliminaryjoining rotating tool A so as to form a one-stroke movement locus(bead).

Friction stir welding steps in the first preparatory joining process ofthe present embodiment will be described in detail.

First, the stirring pin A2 of the preliminary joining rotating tool A isinserted into the start position S_(P) provided at an appropriatelocation on the first tab member 2 while rotating the stirring pin A2counterclockwise and friction stir welding is started, and thepreliminary joining rotating tool A is relatively moved toward the startpoint s2 of the first tab member joining process.

When friction stir welding is performed continuously to the start points2 of the first tab member joining process by relatively moving thepreliminary joining rotating tool A, the preliminary joining rotatingtool A is not withdrawn at the start point s2 but directly shifted tothe first tab member joining process.

In the first tab member joining process, the butting portion J2 betweenthe first tab member 2 and the metal members 1, 1 is friction stirwelded. In detail, a friction stir welding route is set on the seambetween the metal members 1, 1 and the second tab member 2, and byrelatively moving the preliminary joining rotating tool A along thisroute, the butting portion J2 is friction stir welded. In the presentembodiment, friction stir welding is performed continuously from thestart point s2 to the end point e2 of the first tab member joiningprocess without withdrawing the preliminary joining rotating tool Amidway.

When the preliminary joining rotating tool A is rotatedcounterclockwise, fine joining defects may occur on the right in thetraveling direction, therefore, preferably, the positions of the startpoint s2 and the end point e2 of the first tab member joining processare set so that the metal members 1, 1 are positioned on the left in thetraveling direction of the preliminary joining rotating tool A.Accordingly, joining defects hardly occur on the metal member 1 side, sothat a high-quality joined body can be obtained.

When the preliminary joining rotating tool A reaches the endpoint e2 ofthe first tab member joining process, the friction stir welding is notfinished at the endpoint e2 but continued to the start point s1 of thepreliminary joining process, and is directly shifted to the preliminaryjoining process. In the present embodiment, the friction stir weldingroute from the end point e2 of the first tab member joining process tothe start point s1 of the preliminary joining process is set on thefirst tab member 2.

In the preliminary joining process, the butting portion J1 between themetal members 1, 1 is friction stir welded. In detail, a friction stirwelding route is set on the seam between the metal members 1, 1, and byrelatively moving the preliminary joining rotating tool A along thisroute, the butting portion J1 is friction stir welded. In the presentembodiment, friction stir welding is performed continuously from thestart point s1 to the endpoint e1 of the preliminary joining processwithout withdrawing the preliminary joining rotating tool A midway.

When the preliminary joining rotating tool A reaches the endpoint e1 ofthe preliminary joining process, it is directly shifted to the secondtab member joining process. In other words, the process is shifted tothe second tab member joining process without withdrawing thepreliminary joining rotating tool A at the end point e1 of thepreliminary joining process which is the start point s3 of the secondtab member joining process.

In the second tab member joining process, the butting portions J3, J3between the metal members 1, 1 and the second tab member 3 are frictionstir welded. In the present embodiment, the start point s3 of the secondtab member joining process is positioned between the butting portionsJ3, J3, so that a turning point m3 is provided in the friction stirwelding route from the start point s3 to the end point e3 of the secondtab member joining process, and after the preliminary joining rotatingtool A is moved from the start point s3 to the turning point m3 (seeFIG. 16( a)), the preliminary joining rotating tool A is moved from theturning point m3 to the end point e3 (see FIG. 16( b)), whereby frictionstir welding is performed continuously from the start point s3 to theend point e3 of the second tab member joining process. In other words,after the preliminary joining rotating tool A is made to reciprocatebetween the start point s3 and the turning point m3, the preliminaryjoining rotating tool A is moved to the end point e3, whereby frictionstir welding is performed continuously from the start point s3 to theend point e3 of the second tab member joining process. The friction stirwelding route from the start point s3 to the turning point m3 and thefriction stir welding route from the turning point m3 to the end pointe3 are set on the seams between the metal members 1 and the second tabmember 3, respectively.

The positional relation among the start point s3, the turning point m3,and the endpoint e3 is not specifically limited, however, when thepreliminary joining rotating tool A is rotated counterclockwise as inthe present embodiment, preferably, the positions of the start point s3,the turning point m3, and the end point e3 of the second tab memberjoining process are set so that the metal members 1, 1 are positioned onthe left in the traveling direction of the preliminary joining rotatingtool A at least in the friction stir welding route from the turningpoint m3 to the end point e3. In this case, between the start point s3and the turning point m3, the friction stir welding route is set on theseam between the metal members 1 and the second tab member 3 in bothforward and return paths, and preferably, the preliminary joiningrotating tool is moved along this route. Accordingly, in the path fromthe start point s3 to the turning point m3, even if the metal members 1are positioned on the right in the traveling direction of thepreliminary joining rotating tool A and joining defects occur on themetal member 1 side, in subsequent friction stir welding from theturning point m3 to the end point e3, the metal members 1 are positionedon the left in the traveling direction of the preliminary joiningrotating tool A, so that the joining defects are corrected and ahigh-quality joined body can be obtained.

Incidentally, when the preliminary joining rotating tool A is rotatedclockwise, preferably, the positions of the start point, the turningpoint, and the end point of the second tab member joining process areset so that the metal members 1, 1 are positioned on the right in thetraveling direction of the preliminary joining rotating tool A in thefriction stir welding route from the turning point to the endpoint. Indetail, the turning point is provided at the position of the end pointe3 set when the preliminary joining rotating tool A is rotatedcounterclockwise, and the end point is provided at the position of theturning point m3 set when the preliminary joining rotating tool A isrotated counterclockwise although these are not shown.

As shown in FIG. 16( b), when the preliminary joining rotating tool Areaches the end point e3 of the second tab member joining process,friction stir welding is not finished at the end point e3 but continuedto the end position E_(P) provided on the second tab member 3. When thepreliminary joining rotating tool A reaches the end position E_(P), thepreliminary joining rotating tool A is moved up while being rotated towithdraw the stirring pin A2 from the end position E.

Subsequently, the pilot bore forming process is executed. The pilot boreforming process is for forming a pilot bore P1 at the start position offriction stir welding of the first primary welding process. In the pilotbore forming process of the present embodiment, a pilot bore is formedin the butting portion J3 between the metal members 1 and the second tabmember 3.

The method for forming the pilot bore P1 is not limited, and forexample, it can be formed by rotatively inserting a generally knowndrill not shown, and in addition, the pilot bore can also be formed byinserting and extracting a rotating tool with a stirring pin larger thanthe stirring pin A2 (see FIG. 2( a)) of the preliminary joining rotatingtool A and smaller than the stirring pin B2 (see FIG. 2( b)) of theprimary welding rotating tool B while rotating it.

The position of the pilot bore P1 (that is, the start position offriction stir welding of the first primary welding process) is notlimited, either, and it can be formed in the first tab member 2 or thesecond tab member 3, or in the butting portion J2, however, preferably,it is formed on the extension of the seam (boundary) between the metalmembers 1, 1 appearing on the surface 12 side of the metal members 1 asdescribed in the present embodiment or formed at the end portion of theseam between the metal members 1, 1 (that is, the end portion of thebutting portion J1) although this is not shown.

When the pilot bore P1 is formed on the extension of the seam betweenthe metal members 1, 1, as shown in FIG. 17, the end position E_(P) offriction stir welding of the first preparatory process is provided onthe extension and a pull-out hole formed when withdrawing the stirringpin A2 of the preliminary joining rotating tool A is used as a pilotbore, or the pull-out hole is expanded in diameter by using a drill notshown to form a pilot bore. In this case, the pilot bore machiningoperation can be omitted or simplified, so that the operation time canbe shortened.

(3) First Primary Welding Process

When the first preparatory process is finished, the first primarywelding process in which the butting portion J1 between the metalmembers 1, 1 is primarily welded is executed. In the first primarywelding process of the present embodiment, the preliminarily joinedbutting portion J1 is friction stir welded from the surface side of themetal members 1 by using the primary welding rotating tool B shown inFIG. 2( a).

In the first primary welding process, first, as shown in FIG. 18, thestirring pin B2 is inserted into the start position S_(M1) (that is, thepilot bore P1 shown in FIG. 16( b)) while rotating the primary weldingrotating tool B counterclockwise and friction stir welding is started.In the present embodiment, the start position S_(M1) is provided on thebutting portion J3 between the metal member 1 and the second tab member3, when the stirring pin B2 of the primary welding rotating tool B ispress-fitted, a part of the plasticized and fluidized metal flows into afine gap between the metal member 1 and the second tab member 3, andthereafter, the escape of the plasticized and fluidized metal into thegap is reduced, so that joining defects due to insufficient thicknesshardly occur.

When the stirring pin B2 of the primary welding rotating tool B ispress-fitted into the pilot bore P1, a force to separate the metalmember 1 and the second tab member 3 acts, however, the inner cornerportions 3 a, 3 a formed by the metal members 1 and the second tabmember 3 are preliminarily joined by welding, therefore, separation doesnot occur between the metal member 1 and the second tab member 3.

When friction stir welding is performed at one end of the buttingportion J1 between the metal members 1, 1, the primary welding rotatingtool B is made to directly enter the butting portion J1, and byrelatively moving the primary welding rotating tool B along the frictionstir welding route set on the seam between the metal members 1, 1,friction stir welding is performed continuously from the one end to theother end of the butting portion J1. When the primary welding rotatingtool B is relatively moved to the other end of the butting portion J1,it is made to pass across the butting portion J2 while performingfriction stir welding, and then directly and relatively moved toward theend position E_(M1).

When the primary welding rotating tool B reaches the end positionE_(M1), the stirring pin B2 is withdrawn from the end position E_(M1) bymoving up the primary welding rotating tool B while rotating it. Whenthe stirring pin B2 is withdrawn upward at the end position E_(M1), apull-out hole Q1 having substantially the same shape as the stirring pinB2 is inevitably formed, and this remains in the present embodiment.

In the present embodiment, the case where the start position of frictionstir welding of the first primary welding process is provided on thebutting portion J3 is illustrated, however, as shown in FIG. 19( a), itmay be provided on the second tab member 3, or on the first tab member 2as shown in FIG. 19( b).

(4) First Repairing Process

When the first primary welding process is finished, the surfaceplasticized region W1 formed on the metal members 1 through the firstprimary welding process is friction stir welded through the firstrepairing process. In the first repairing process of the presentembodiment, as shown in FIG. 20( a) and FIG. 20( b), at least the firstrepairing region R1 and the second repairing region R2 in the surfaceplasticized region W1 are friction stir welded by using a repairingrotating tool C smaller than the primary welding rotating tool B.

The friction stir welding on the first repairing region R1 is performedfor dividing a tunnel-like defect which may be formed along thetraveling direction of the primary welding rotating tool B (see FIG. 2(b)). In the present embodiment in which the primary welding rotatingtool B is rotated counterclockwise, a tunnel-like defect may occur onthe right in the traveling direction, so that the first repairing regionR1 is preferably set so as to include at least the upper portion of thesurface plasticized region W1 positioned on the right in the travelingdirection in a plan view.

Friction stir welding on the second repairing region R2 is performed fordividing an oxide film (oxide film formed on the contact surface 21between the lateral sides 14 of the metal members 1 and the first tabmember 2) involved in the surface plasticized region W1 when the primarywelding rotating tool B passes across the butting portion J2. As in thepresent embodiment, when the end position E_(M1) of friction stirwelding of the primary welding process is provided on the first tabmember 2 and the primary welding rotating tool B (see FIG. 2( b)) isrotated counterclockwise, there is every possibility that an oxide filmis involved in the upper portion of the surface plasticized region W1 onthe left in the traveling direction, so that the second repairing regionR2 is preferably set so as to include at least the upper portion of thesurface plasticized region W1 positioned on the left in the travelingdirection in a plan view in the surface plasticized region W1 adjacentto the first tab member 2.

In the first repairing process, as shown in FIG. 20( b), by moving onerepairing rotating tool to form a one-stroke movement locus (bead), thefirst repairing region R1, the second repairing region R2, and the thirdrepairing region R3 are successively friction stir welded. In thepresent embodiment, the case where the first repairing region R1 and thesecond repairing region R2 are friction stir welded in this order isillustrated, however, this is not intended to limit the friction stirwelding order.

Friction stir welding steps of the first repairing process will bedescribed in detail with reference to FIG. 20( b).

First, the stirring pin of the repairing rotating tool C is inserted(press-fitted) to the start position S_(R) provided on an appropriatelocation on the metal member 1 and friction stir welding is started, andthe first repairing region R1 (see FIG. 20( a)) is friction stir welded.In the present embodiment, the start position S_(R) is provided near thestart position S_(M1) (see FIG. 18) of friction stir welding of theprimary welding process, and a turning point M_(R) is provided on theopposite side of the end position E_(R) across the start position S_(R),and the repairing rotating tool C is relatively moved toward the turningpoint M_(R) and then turned around at the turning point M_(R), andrelatively moved along the butting portion J1 (see FIG. 18), whereby thefirst repairing region R1 (see FIG. 20( a)) is friction stir welded. Byperforming friction stir welding from the start position S_(R) to theturning point M_(R), an oxide film involved when the stirring pin B2 ofthe primary welding rotating tool B is inserted into the start positionS_(M1) (see FIG. 18) can be divided.

When friction stir welding on the first repairing region R1 is finished,the repairing rotating tool C is directly moved to the second repairingregion R2 without being withdrawn to friction stir weld the secondrepairing region R2. When the second repairing region R2 is larger thanthe region which can be friction stir welded by the repairing rotatingtool C, the repairing rotating tool C is turned around several timeswhile shifting the friction stir welding route.

When the friction stir welding on the second repairing region R2 isfinished, the repairing rotating tool C is moved to the end positionE_(R), and the stirring pin C2 is withdrawn from the end position E_(R)by moving up the repairing rotating tool C while rotating it.

(5) First Traverse Repairing Process

When the first repairing process is finished, as shown in FIG. 21, bymoving the traversing rotating tool D so as to traverse the surfaceplasticized region W1 a plurality of times, the first traverse repairingprocess in which the surface plasticized region W1 is friction stirwelded is executed.

The friction stir welding route of the first traverse repairing processis set so that a plurality of re-plasticized regions W3, W3 . . . arespaced from each other on the friction stir welding route (center lineof the surface plasticized region W1) of the first primary weldingprocess, and in the present embodiment, includes a plurality of traverseroutes F1, F1 . . . traversing the surface plasticized region W1 andtransfer routes F2, F2 . . . connecting end portions on the same side ofthe traverse routes F1, F1 adjacent to each other.

The traverse routes F1 cross the friction stir welding route of thefirst primary welding process at right angles. The transfer routes F2are provided on the right or left of the surface plasticized region W1and parallel to the friction stir welding route of the first primarywelding process.

Friction stir welding steps of the first traverse repairing process willbe described in detail.

In the present embodiment, by moving one traversing rotating tool Dzigzag to form a one-stroke movement locus (bead), friction stir weldingis also performed continuously from the start position S_(C)to the endposition E_(C) of friction stir welding.

In the first traverse repairing process, first, the stirring pin D2 (seeFIG. 9) of the traversing rotating tool D is inserted (press-fitted) tothe start position S_(C) provided at an appropriate location on themetal members 1 and friction stir welding is started, and friction stirwelding is performed continuously along the first traverse route F1.When it reaches the endpoint e10 of the first traverse route F1, thetraveling direction of the traversing rotating tool D is changed and thetraversing rotating tool D is moved along the transfer route F2 tocontinuously friction stir weld the metal lateral to the surfaceplasticized region W1. When the traversing rotating tool D reaches thestart point S10 of the second traverse route F1, the traveling directionof the traversing rotating tool D is changed and the traversing rotatingtool D is moved along the second traverse route F1 to friction stir weldthe surface plasticized region W1 continuously. By repeating theabove-described steps, when the traversing rotating tool D reaches theend point e10 of the last traverse route F1, the traversing rotatingtool D is moved to the end position E_(C), and the stirring pin D2 (seeFIG. 9) is withdrawn from the end position E_(C) by moving up thetraversing rotating tool D while rotating it.

When the first traverse repairing process is finished, burrs formedduring friction stir welding in the first preparatory joining process,the first primary welding process, the first repairing process, and thefirst traverse repairing process are removed, and the metal members 1, 1are turned over so that the back surfaces thereof turn up.

(6) Second Preparatory Process

After the metal members 1, 1 are turned over, the second preparatoryprocess is executed. The second preparatory process of the presentembodiment includes a pilot bore forming process in which a pilot bore(not shown) is formed at a start position of friction stir welding ofthe second primary welding process.

(7) Second Primary Welding Process

When the second preparatory process is finished, as shown in FIG. 22,the second primary welding process is executed to friction stir weld thebutting portion J1 from the back surface 13 side of the metal members 1by using the primary welding rotating tool B used in the first primarywelding process. When friction stir welding the butting portion J1,friction stir welding is performed while the stirring pin B2 of theprimary welding rotating tool B is made to get into the surfaceplasticized region formed through the first primary welding process (seeFIG. 10( b)).

In the second primary welding process of the present embodiment, thefriction stir welding route is set so as to avoid the pull-out hole Q1remaining in the first primary welding process, and by moving thestirring pin inserted into the pilot bore (start position S_(M2)) notshown to the end position E_(M2) without withdrawing it midway, frictionstir welding is performed continuously from the start position S_(M2) tothe end position E_(M2). The shortest distance d₁ in a plan view betweenthe end position E_(M1) (center of the pull-out hole Q1) of frictionstir welding of the first primary welding process and the movement locusof the primary welding rotating tool B in the second primary weldingprocess is secured not less than the outer diameter of the shoulder B1of the primary welding rotating tool B.

In addition, steps, etc., of the second primary welding process of thepresent embodiment are the same as those of the second primary weldingprocess of the first embodiment described above, so that detaileddescription thereof is omitted.

(8) Second Repairing Process

When the second primary welding process is finished, the secondrepairing process is executed to friction stir weld the back plasticizedregion W2 formed on the metal members 1 through the second primarywelding process. The second repairing process is the same as the firstrepairing process described above except that the friction stir weldingis performed from the back surface 13 side of the metal members 1, sothat detailed description of the second repairing process is omitted.

(9) Second Traverse Repairing Process

When the second repairing process is finished, the second traverserepairing process is executed to friction stir weld the back plasticizedregion W2 formed on the metal members 1 through the second primarywelding process. The second traverse repairing process is the same asthe first traverse repairing process except that the friction stirwelding is performed from the back surface 13 side of the metal members1, so that detailed description of the second traverse repairing processis omitted.

When the second traverse repairing process is finished, burrs formedduring friction stir welding in the second preparatory process, thesecond primary welding process, the second repairing process, and thesecond traverse repairing process are removed, and further, the firsttab member 2 and the second tab member 3 are cut off.

Through the above-described processes (1) to (9), even when extremelythick metal members 1, 1 whose thicknesses are more than 40 (mm) arejoined, airtightness and water-tightness at the joined portion can beimproved.

In the second embodiment, the case where the second tab member 3 isdisposed at the inner corner portion of the metal members 1, 1 buttedagainst each other in an L shape is illustrated, however, it is alsoallowed that, as shown in FIG. 23, the first tab member 2 is disposed atthe inner corner portions of the metal members 1, 1 and the second tabmember 3 is disposed on the opposite side across the butting portion J1.

In this case, it is allowed that the start point s2 and the turningpoint m2 of the first tab member joining process are provided on theseams between the metal members 1 and the first tab member 2, the startpoint s1 (inner corner portion of the metal members 1, 1) of thepreliminary joining process between the start point s2 and the turningpoint m2 is set as the end point e2, and by moving the preliminaryjoining rotating tool A from the turning point m2 to the endpoint e2after moving it from the start point s2 to the turning point m2, thebutting portions J2, J2 may be friction stir welded.

When the preliminary joining rotating tool A is rotated clockwise,preferably, the positions of the start point s2, the turning point m2,and the end point e2 of the first tab member joining process are set sothat the metal members 1, 1 are positioned on the right in the travelingdirection of the preliminary joining rotating tool A at least in thefriction stir welding route from the start point s2 to the turning pointm2. Incidentally, when the preliminary joining rotating tool A isrotated counterclockwise, preferably, the positions of the start point,the turning point, and the end point of the first tab member joiningprocess are set so that the metal members 1, 1 are positioned on theleft in the traveling direction of the preliminary joining rotating toolA at least in the friction stir welding route from the start point tothe turning point although these are not shown.

Similarly, in the second tab member joining process, it is also allowedthat the turning point m3 and the end point e3 are provided on the seambetween the metal members 1 and the second tab member 3, the end pointe1 of the preliminary joining process between the turning point m3 andthe endpoint e3 is set as the start point s3, and by moving thepreliminary joining rotating tool A from the turning point m3 to the endpoint e3 after moving it from the start point s3 to the turning pointm3, the butting portion J3 may be friction stir welded.

When the preliminary joining rotating tool A is rotated clockwise,preferably, the positions of the start point s3, the turning point m3,and the end point e3 of the second tab member joining process are set sothat the metal members 1, 1 are positioned on the right in the travelingdirection of the preliminary joining rotating tool A at least in thefriction stir welding route from the turning point m3 to the end pointe3. Incidentally, when the preliminary joining rotating tool A isrotated counterclockwise, preferably, the positions of the start point,the turning point, and the end point of the second tab member joiningprocess are set so that the metal members 1, 1 are positioned on theleft in the traveling direction of the preliminary joining rotating toolA at least in the friction stir welding route from the turning point tothe end point although these are not shown.

Examples

Detailed joining conditions of the above-described joining method areillustrated in Table 1 to Table 2. Table 1 shows the kinds andthicknesses of alloys of the metal members 1, the first tab member 2,and the second tab member 3, dimensions of the rotating tools, anddimensions of the pilot bores, and Table 2 shows the rotating speeds,insertion speeds, and movement speeds of the rotating tools in therespective processes described above. In the present example, thejoining conditions of the first primary welding process and the secondprimary welding process are the same, and the joining conditions of thefirst repairing process and the second repairing process are the same.In Table 1, “JIS 5052-O” is a sign showing a kind of an aluminum alloydefined in Japanese Industrial Standards, and means an annealed aluminumalloy (Al—Mg alloy) containing 2.2% to 2.8% of Mg obtained. “C1020” is asign showing a kind of copper (copper alloy) defined in JapaneseIndustrial Standards, and means high-purity oxygen-free copper (not lessthan 99.96% of Cu).

TABLE 1 No. 1 No. 2 No. 3 Metal Kind of alloy Alumi- Alumi- Coppermembers 1 num num (C1020) First tab alloy alloy member 2 (JIS (JISSecond tab 5052-O) 5052-O) member 3 Thickness t (mm) 40 105 40Preliminary Shoulder Outer diameter 20.0 20.0 20.0 joining A1 X₁ (mm)rotating tool Stirring Maximum outer 8.0 10.0 9.0 A pin A2 diameter X₂(mm) Minimum outer 6.0 8.0 6.0 diameter X₃ (mm) Length L_(A) (mm) 4.010.0 5.0 Primary Shoulder Outer diameter 30.0 55.0 32.5 welding B1 Y₁(mm) rotating tool Stirring Maximum outer 16.0 36.0 16.0 B pin B2diameter Y₂ (mm) Minimum outer 8.0 18.0 8.0 diameter Y₃ (mm) Length L₁(mm) 21.0 57.0 21.0 Pilot bore Maximum bore 10.0 20.0 14.0 P1 diameterZ₁ (mm) Minimum bore 10.0 20.0 14.0 diameter (mm) Depth Z₂ (mm) 12.054.0 12.0 Repairing Shoulder Outer diameter 20.0 30.0 20.0 rotating toolC1 (mm) C (same for Stirring Maximum outer 10.0 16.0 9.0 traversing pinC2 diameter (mm) rotating tool Minimum outer 8.0 8.0 6.0 D) diameter(mm) Length (mm) 8.0 21.0 7.0

TABLE 2 No. 1 No. 2 No. 3 Preliminary Rotating For insertion 700 6001000 joining speed For movement 700 600 1000 rotating (rpm) Forwithdrawal 700 600 1000 tool A Insertion speed 40 40 50 (moving-downspeed) (mm/min) Movement speed (feed rate) 200 150 100 (mm/min)Withdrawal speed 60 60 60 (moving-up speed) (mm/min) Primary RotatingFor insertion 400 150 500 welding speed For movement 400 90 500 rotating(rpm) For withdrawal 400 150 500 tool B Insertion speed 40 10 30(moving-down speed) (mm/min) Movement speed (feed rate) 100 50 80(mm/min) Withdrawal speed 60 20 60 (moving-up speed) (mm/min) RepairingRotating For insertion 600 400 1000 rotating speed For movement 600 4001000 tool C (rpm) For withdrawal 600 400 1000 (same for Insertion speed40 40 50 traversing (moving-down speed) rotating (mm/min) tool D)Movement speed (feed rate) 150 100 100 (mm/min) Withdrawal speed 60 6060 (moving-up speed) (mm/min)

1. A joining method comprising: a butting step in which metal members tobe joined are butted against each other to form a butting portionbetween the metal members; a tab member disposing step in which a firsttab member and a second tab member are disposed on both sides of thebutting portion between the metal members; a first tab member joiningstep in which the butting portion between the metal members and thefirst tab member is friction stir welded from a first surface; apreliminary joining step in which the butting portion between the metalmembers is preliminarily friction stir welded from the first surface; asecond tab member joining step in which the butting portion between themetal members and the second tab member is friction stir welded from thefirst surface; a first primary welding step in which the butting portionpreliminarily joined between the metal members is friction stir weldedusing a first rotating tool with a first stirring pin from the firstsurface of the metal members, and the first rotating tool used in thefirst primary welding step reaches an end position on the first surfaceof the first tab member or the second tab member where the firstrotating tool is moved up to withdraw the first stirring pin from theend position; and a second primary welding step in which the buttingportion between the metal members is friction stir welded using a secondrotating tool with a second stirring pin from a back surface of themetal members, opposite said first surface, wherein, in the secondprimary welding step, friction stir welding is performed while insertingthe second stirring pin of the second: rotating tool into a plasticizedregion formed through the first primary welding step.
 2. A joiningmethod comprising: a butting step in which metal members to be joinedare butted against each other to form a butting portion between themetal members; a tab member disposing step in which a first tab memberand a second tab member are disposed on both sides of the buttingportion between the metal members; a first tab member joining step inwhich the butting portion between the metal members and the first tabmember is friction stir welded from a back surface; a preliminaryjoining step in which the butting portion between the metal members ispreliminarily friction stir welded from the back surface; a second tabmember joining step in which the butting portion between the metalmembers and the second tab member is friction stir welded from the backsurface; a first primary welding step in which the butting portionbetween the metal members is friction stir welded from a first surfaceof the metal members, opposite said back surface; and a second primarywelding step in which the butting portion preliminarily joined betweenthe metal members is friction stir welded using a second rotating toolwith a second stirring pin from the back surface of the metal members,and the second rotating tool used in the second primary welding stepreaches an end position on the back surface of the first tab member orthe second tab member where the second rotating tool is moved up towithdraw the second stirring pin from the end position, wherein, in thesecond primary welding step, friction stir welding is performed whileinserting the second stirring pin of the second rotating tool into aplasticized region formed through the first primary welding step.
 3. Thejoining method according to claim 1, wherein a sum of a length of thefirst stirring pin of the first rotating tool used in the first primarywelding step and a length of the second stirring pin of the secondrotating tool used in the second primary welding step is at least equalto a thickness of the metal members at the butting portion.
 4. Thejoining method according to claim 2, wherein a sum of a length of thefirst stirring pin of the first rotating tool used in the first primarywelding step and a length of the second stirring pin of the secondrotating tool used in the second primary welding step is at least equalto a thickness of the metal members at the butting portion.
 5. Thejoining method according to claim 1, wherein a length of the firststirring pin of the first rotating tool to be used in the first primarywelding step is set to not less than ½ of a thickness of the metalmembers at the butting portion, and a length of the second stirring pinof the second rotating tool to be used in the second primary weldingstep is set to not less than ½ of the thickness of the metal members atthe butting portion.
 6. The joining method according to claim 2, whereina length of the first stirring pin of the first rotating tool to be usedin the first primary welding step is set to not less than ½ of athickness of the metal members at the butting portion, and a length ofthe second stirring pin of the second rotating tool to be used in thesecond primary welding step is set to not less than ½ of the thicknessof the metal members at the butting portion.
 7. The joining methodaccording to claim 5, wherein the length of the second stirring pin ofthe second rotating tool to be used in the second primary welding stepis set to be equal to the length of the first stirring pin of the firstrotating tool to be used in the first primary welding step.
 8. Thejoining method according to claim 6, wherein the length of the secondstirring pin of the second rotating tool to be used in the secondprimary welding step is set to be equal to the length of the firststirring pin of the first rotating tool to be used in the first primarywelding step.
 9. The joining method according to claim 1, satisfying arelation of 1.01≦(L₁+L₂)/t≦1.10, where L₁ is a length of the firststirring pin of the first rotating tool to be used in the first primarywelding step, L₂ is a length of the second stirring pin of the secondrotating tool to be used in the second primary welding step, and t is athickness of the metal members at the butting portion.
 10. The joiningmethod according to claim 2, satisfying a relation of1.01≦(L₁+L₂)/t≦1.10, where L₁ is a length of the first stirring pin ofthe first rotating tool to be used in the first primary welding step, L₂is a length of the second stirring pin of the second rotating tool to beused in the second primary welding step, and t is a thickness of themetal members at the butting portion.
 11. The joining method accordingto claim 1, wherein a length of the first stirring pin of the firstrotating tool to be used in the first primary welding step is set to notmore than ¾ of a thickness of the metal members at the butting portion,and a length of the second stirring pin of the second rotating tool tobe used in the second primary welding step is set to not more than ¾ ofthe thickness of the metal members at the butting portion.
 12. Thejoining method according to claim 2, wherein a length of the firststirring pin of the first rotating tool to be used in the first primarywelding step is set to not more than ¾ of a thickness of the metalmembers at the butting portion, and a length of the second stirring pinof the second rotating tool to be used in the second primary weldingstep is set to not more than ¾ of the thickness of the metal members atthe butting portion.
 13. The joining method according to claim 1,wherein a friction stir welding route in the second primary welding stepis set so as to avoid a pull-out hole formed by withdrawing the firststirring pin at an end position of friction stir welding of the firstprimary welding step.
 14. The joining method according to claim 2,wherein a friction stir welding route in the second primary welding stepis set so as to avoid a pull-out hole formed by withdrawing the firststirring pin at an end position of friction stir welding of the firstprimary welding step.
 15. The joining method according to claim 13,wherein a shortest distance between the end position of friction stirwelding of the first primary welding step and a movement locus of thesecond stirring pin of the second rotating tool used in the secondprimary welding step is set to not less than an outer diameter of ashoulder of the second rotating tool to be used in the second primarywelding step.
 16. The joining method according to claim 14, wherein ashortest distance between the end position of friction stir welding ofthe first primary welding step and a movement locus of the secondstirring pin of the second rotating tool used in the second primarywelding step is set to not less than an outer diameter of a shoulder ofthe second rotating tool to be used in the second primary welding step.17. The joining method according to claim 1, wherein in the preliminaryjoining step, a preliminary joining rotating tool smaller than the firstrotating tool to be used in the first primary welding step is used. 18.The joining method according to claim 2, wherein in the preliminaryjoining step, a preliminary joining rotating tool smaller than thesecond rotating tool to be used in the second primary welding step isused.
 19. The joining method according to claim 1, further comprising awelding step in which the first tab member and the second tab member arepreliminarily joined to the metal members by welding before the firstprimary welding step.
 20. The joining method according to claim 2,further comprising a welding step in which the first tab member and thesecond tab member are preliminarily joined to the metal members bywelding before the second primary welding step.
 21. The joining methodaccording to claim 19, wherein in the welding step, inner cornerportions formed by the metal members and the first tab member arewelded, and inner corner portions formed by the metal members and thesecond tab member are welded.
 22. The joining method according to claim20, wherein in the welding step, inner corner portions formed by themetal members and the first tab member are welded, and inner cornerportions formed by the metal members and the second tab member arewelded.
 23. The joining method according to claim 1, wherein before thefirst primary welding step, the metal members are fixedly locked on aplatform of a friction stir welding machine, using a jig, whereby thefirst primary welding step is performed with the metal members heldimmovably on said platform.
 24. The joining method according to claim 2,wherein before the second primary welding step, the metal members arefixedly locked on a platform of a friction stir welding machine, using ajig, whereby the second primary welding step is performed with the metalmembers held immovably on said platform.
 25. The joining methodaccording to claim 1, wherein after the first primary welding step,burrs formed during friction stir welding are removed, and the metalmembers are then turned over so that the back surfaces thereof turn up.26. The joining method according to claim 2, wherein after the secondprimary welding step, burrs formed during friction stir welding areremoved, and the metal members are then turned over so that the firstsurfaces thereof turn up.